UNITED STATES DISTRICT COURT

NORTHERN DISTRICT OF ILLINOIS

EASTERN DIVISION

UNITED STATES OF AMERICA

v.

MARTIN A. CROSSLEY

No. 20 CR 369 

Judge Martha M. Pacold

GOVERNMENT’S RESPONSE IN OPPOSITION TO 

DEFENDANT’S MOTION IN LIMINE TO BAR EXPERT TESTIMONY

The UNITED STATES OF AMERICA, by its attorney, JOHN R. LAUSCH, JR., 

United States Attorney for the Northern District of Illinois, hereby provides its 

response in opposition to defendant Martin Crossley’s motion in limine to bar the 

expert testimony of Federal Bureau of Investigation Special Agent Jeremy Bauer. 

Dkt. 57. 

BACKGROUND

At trial, the government intends to call Federal Bureau of Investigation (FBI) 

Special Agent (SA) Jeremy Bauer as an expert witness in the field of historical cell 

site analysis. The government provided notice to the defense of this expert witness 

testimony on May 6, 2021 (expert notice and Curriculum Vitae, attached as Exhibits 

A and B), and May 11, 2021 (supplemental expert disclosure with expert report, 

attached as Exhibit C). SA Bauer is currently finalizing his anticipated trial 

presentation, which is based on the conclusions in the previously disclosed expert 

materials, and a copy of his trial presentation will be produced to the defense upon 

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receipt by the United States. The government has also previously produced the call 

detail records for the cell phone number that SA Bauer analyzed.

At trial, SA Bauer is expected to testify about the attached exhibits that he 

prepared based on the cell site records for the defendant’s telephone number. These 

exhibits are maps reflecting: (1) the location of T-Mobile cell towers in the Illinois and 

Wisconsin area; (2) the location of the specific cell towers utilized by the subject 

phones on material dates, and (3) the location of other places that are pertinent to 

the charged crime.

Also included among the exhibits is a map that displays an estimated distance 

from cell tower to cell phone based on timing advance data.1 The map does not purport 

to pinpoint the location of the phone but rather shows a general area where the phone 

would be expected to be based on information received from the cell phone service 

provider. This is illustrated by a wide arc intended to show the approximate phone 

location as being within +/- 100 meters of an arc of distance estimated based on the 

service provider information concerning timing advance data, cell site location, and 

which cell tower face was used:

1 As SA Bauer will testify, timing advance data is a measurement derived from the amount of 

time it takes for a signal to travel from a cell phone to a tower and return. 

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Defendant seeks exclusion of SA Bauer’s testimony on grounds that it is not 

based on reliable principles and methods and could lead to juror confusion. Contrary 

to the defendants’ claim, the evidence that the government intends to offer –

historical cell site analysis – is scientifically valid, probative, reliable and relevant. It 

will also assist the trier of fact in deciding the facts at issue in this case. The United 

States respectfully submits that this Court should exercise its broad discretion to 

allow expert testimony concerning historical cell tower analysis. Further, the United 

States respectfully submits that it is unnecessary to conduct a pretrial hearing on the 

requirements of Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), 

for this type of evidence and testimony.

SA Bauer has been qualified and testified as an expert witness on historical 

cell site analysis twice over the past two years in this District. Ex. B. SA Bauer has 

also testified as an expert in historical cell site analysis two times in state court. Id.

SA Bauer has been an FBI Agent for 5 years and served the FBI in other capacities 

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for an additional 6 years before that. He has received over 300 hours of training and 

instruction in the use of cellular phones and investigations. Those training include 

trainings from T-Mobile, the cellular service provider at issue in this case. He has 

instructed approximately 500 officers in basic techniques for utilizing cell phones in 

investigations. He has also used historical cell site data on numerous occasions to 

locate individuals in other FBI investigations.

If called to testify as an expert in the field of historical cell site analysis, the 

government expects SA Bauer would testify about basic cell phone operation and how 

it is possible to learn the general location of a cell phone receiving and making cell 

phone calls. SA Bauer will not testify about the defendant’s cell phone’s precise 

location using this method of analysis. He would testify that he was provided with 

certain dates and times relevant to this case and that he reviewed these dates and 

times in connection with the cell phone records. He would testify that he was able to 

determine which T-Mobile cell towers the cell phone at issue in this case utilized on

the material dates.

The principles and methodology that SA Bauer proposes to rely upon in this 

case have been accepted by judges throughout the United States, including the 

Northern District of Illinois. Defendant has not cited a single case in which a court 

has concluded that Daubert principles should preclude the admission of the type of 

testimony that Agent Bauer will present in this case. Indeed, as discussed further 

below, the lone case cited by defendant that directly addressed the reliability of 

historic cell site analysis was a Seventh Circuit opinion that affirmed the district 

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court’s admission of such analysis. Mot. at 8 (citing United States v. Hill, 818 F.3d 

289 (7th Cir. 2016)). 

ARGUMENT

A. Legal Standard 

The admissibility of expert testimony is governed by Federal Rule of Evidence 

702 and the framework established by the Supreme Court in Daubert. Specifically, 

expert testimony is admissible at trial pursuant to Fed.R.Evid. 702 as set forth below:

A witness who is qualified as an expert by knowledge, skill, 

experience, training, or education may testify in the form 

of an opinion or otherwise if: (a) the expert's scientific, 

technical, or other specialized knowledge will help the trier 

of fact to understand the evidence or to determine a fact in 

issue; (b) the testimony is based on sufficient facts or data;

(c) the testimony is the product of reliable principles and 

methods; and (d) the expert has reliably applied the 

principles and methods to the facts of the case.

Fed.R.Evid. 702. 

First, to determine whether the proposed expert testimony is reliable, courts

determine “whether the expert is qualified in the relevant field” and “examine the

methodology the expert has used in reaching his conclusions.” Smith v. Ford Motor 

Co., 215 F.3d 713, 718 (7th Cir. 2000), citing Kumho, 526 U.S. at 153. First, an expert 

witness may be qualified by “knowledge, skill, experience, training, or education.” 

Fed. R. Evid. 702. An expert must have “extensive academic and practical expertise” 

or knowledge that “is based on experience.” Smith, 215 F.3d at 718, quoting Bryant 

v. City of Chicago, 200 F.3d 1092, 1098 (7th Cir. 2000), and Walker v. Soo Line 

Railroad Co., 208 F.3d 581, 591 (7th Cir. 2000) (“Rule 702 specifically contemplates 

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the admission of testimony by experts whose knowledge is based on experience”). See 

also Fed. R. Evid. 702, 2000 Advisory Committee Note (“[i]n certain fields, experience 

is the predominant, if not the sole, basis for a great deal of reliable expert testimony.”) 

and Metavante Corp. v. Emigrant Savings Bank, 619 F.3d 748, 761 (7th Cir. 2010) 

(“An expert’s testimony is not unreliable simply because it is founded on his 

experience rather than on data . . ..”).

Second, a court in its role as a gatekeeper must examine the expert’s 

methodology and determine whether it is “sound.” Smith, 215 F.3d at 719. Trial 

courts “in all cases of proffered expert testimony must find that it is properly 

grounded, well-reasoned, and not speculative before it can be admitted.” Fed. R. Evid. 

702, 2000 Advisory Committee Notes. Trial courts, however, do not examine the 

“soundness of the factual underpinnings of the expert’s analysis,” “the correctness of 

the expert’s conclusions,” or whether the expert is credible to determine the 

admissibility of the expert’s testimony. Smith, 215 F.3d. at 718- 19. Those are factual 

matters reserved for the trier of fact “to determine after opposing counsel has been 

provided the opportunity to cross-examine the expert regarding his conclusions and 

the facts which they are based.” Id. at 719.

The Supreme Court in Daubert listed four non-exclusive factors that are 

helpful to determine the reliability of scientific or technical testimony: (1) whether 

the scientific theory or technique can be (and has been) tested; (2) whether the theory 

or technique has been subjected to peer review and publication; (3) whether a 

particular technique has a known potential rate of error; and (4) whether the theory 

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or technique is generally accepted in the relevant scientific community. Daubert, 509 

U.S. at 593-95. The Court noted that these factors do not constitute a “definitive 

checklist or test,” and that “[m]any factors will bear on the inquiry” that involves “a 

preliminary assessment of whether the reasoning or methodology underlying the 

testimony is scientifically valid and of whether that reasoning or methodology 

properly can be applied to the facts in issue.” Id. at 592-93. The inquiry to be 

undertaken by a trial court is “a flexible one” focusing on the “principles and 

methodology” employed by an expert, not on the conclusions reached. Id. at 594-95. 

Expert testimony, like all other admissible evidence, is subject to testing by 

“[v]igorous cross-examination, presentation of contrary evidence, and careful 

instruction on the burden of proof.” Id. at 596.

In Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137 (1999), the Supreme Court 

extended Daubert to include expert testimony involving technical and other 

specialized knowledge. Kumho Tire, 526 U.S. at 141. The Court held that a trial court 

“may consider one or more of the specific factors that Daubert mentioned when doing 

so will help to determine that testimony’s reliability.” Id. The Court again noted that 

a trial court considering the admissibility of expert testimony exercises a gatekeeping function to assess whether the proffered evidence is sufficiently reliable and 

relevant. Id.

Kumho Tire also made it clear that a trial court is not required to conduct a 

pre-trial hearing based on a Daubert challenge. Id. at 152. A trial court has 

considerable latitude in deciding whether or when special briefing or other 

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proceedings are needed to investigate challenged reliability of proposed expert 

testimony. Id. at 152-53. Pretrial evidentiary hearings are particularly unnecessary 

when expert testimony is based on well-established principles.

B. Numerous Courts Have Affirmed the Reliability of Historic Cell 

Site Analysis for Establishing the General Location of a 

Telephone.

Testimony about cell phone technology and the ability to determine the general 

area where calls are placed and received has been admitted in courts throughout the 

country. As noted above, SA Bauer has been qualified to offer expert testimony on

historical cell site analysis multiple times. The science behind how cell phones work 

is accepted in the scientific and legal community. The defendant has not cited, and 

the government has not found, any cases in which a court has excluded a qualified 

expert from providing testimony on this topic for the purpose of establishing the 

general location of a cellular phone. Rather, courts routinely admit expert testimony 

of the kind SA Bauer is prepared to provide. 

This Court need look no further than United States v. Hill, 818 F.3d 289 (7th

Cir. 2016)—the only case cited by defendant that concerned historic cell site 

analysis—for an endorsement of the admissibility of cell site analysis. While Hill

cautioned against the possibility of overselling the precision of historic cell site data, 

it concluded that “[h]istorical cell-site analysis can show with sufficient reliability 

that a phone was in a general area, especially in a well-populated one.” Id. at 298. 

Likewise, while the “mathematical error rate [of historic cell site analysis] has not 

been calculated ... the technique has been subjected to publication and peer criticism, 

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if not peer review.” Id. Ultimately, the Seventh Circuit held the admission of historic 

cell site testimony by a government expert was appropriate given the expert’s 

testimony acknowledging the limitations of the analysis. Id; see also United States v. 

Lewisbey, 843 F.3d 653, 659 (7th Cir. 2016) (Holding that “[u]sing call records and 

cell towers to determine the general location of a phone at specific times is a wellaccepted, reliable methodology.”).

Many other courts have endorsed the reliability of the type of testimony SA 

Bauer will present. United States v. Schaffer, 439 Fed.Appx. 344, 2011 WL 3820957, 

at 2 (5th Cir. Aug. 24, 2011) (affirming admission of cell site analysis and noting that 

cell site analysis is “neither untested nor unestablished.”); United States v. MachadoErazo, 2013 WL 2932399 (D.D.C. June 17, 2013) (historical cell site analysis was 

based on “scientific knowledge” and thus “admissible under Daubert”); United States 

v. Davis, 2013 WL 2156659 (S.D. Fla. May 17, 2013); United States v. Reynolds, 2013 

WL 2480684, at *5 (E.D. Mich. June 10, 2013) (“the ability to determine the general 

area where calls are placed and received has been widely accepted by federal courts”

and historical cell site analysis was “the product of reliable principles and methods”); 

United States v. Henderson, No. CR 10-117 BDB, 2011 WL 6016477, at *4-*5 (N.D. 

Okla. Dec. 2, 2011); United States v. Allums, No. 02:08-CR-30 TS, 2009 WL 806748, 

at *2-*3 (D. Utah March 24, 2009); United States v. Benford, 2010 WL 2346305 (N.D. 

Ind.) (denying motion to exclude cell site analysis that would be introduced for 

purpose of revealing the general vicinity of where a cell phone was located). 

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Defendant argues that cell site analysis is unreliable given the lack of a known 

error rate for records provided by the cell service provider and T-Mobile’s disclaimer 

that “T-Mobile does not conduct systematic testing of TrueCall data to ascertain the 

reliability or consistency of the location data it collects.“ So long as there are sufficient

other indicia of reliability, lack of known error rate is not grounds for excluding an 

expert because “the Rule 702 test is a flexible one, and no single factor is either 

required in the analysis or dispositive as to its outcome.” Smith, 215 F.3d 713, 719 

(7th Cir. 2000). As set forth above, numerous courts have accepted historic cell site 

analysis as reliable for the purpose that it will be introduced in this trial: determining 

the general location of a phone at a given time. As SA Bauer will testify, historic cell 

site analysis is known to be reliable in part due to the plentiful investigations where 

historic cell cite was used to track down phones or persons associated with phones.

SA Bauer will also speak to his personal involvement in investigations where historic 

cell site was used for this purpose. 

SA Bauer’s testimony will acknowledge T-Mobile’s disclaimer concerning

testing the reliability or consistency of its location data. The government anticipates 

that SA Bauer will further testify that, based on his training and experience, the 

accuracy of timing advance data—the measure from which location estimates 

derive—is important to the overall functionality of cellular network services. 

Defendant also argues that cell phone analysis is unreliable because phones 

connect to towers with the strongest signal, which is not always the closest tower. 

That observation is true but not a basis for exclusion. The court in United States v. 

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Jones, 918 F. Supp. 2d 1 (D.D.C. 2013) encountered and rejected such an argument 

in a motion to exclude historic cell site analysis: “the mere existence of factors 

affecting cell signal strength that the expert may not have taken into account goes to 

the weight of the expert's testimony and is properly the subject of cross-examination, 

but does not render the fundamental methodology of cell site analysis unreliable.” Id. 

at 5. The court went on to find the agent’s proposed testimony to be “based on reliable 

methodology” and noted “the use of cell phone location records to determine the 

general location of a cell phone has been widely accepted by numerous federal courts.” 

Id. 

Numerous cases have reached the same conclusion: historical cell site 

testimony is sufficiently reliable to be admissible. The defendant’s motion is focused 

largely on the concern that SA Bauer will leave jurors with an impression that 

historical cell site analysis allows for a more precise estimate of phone location than 

it is truly capable of. This will be addressed through SA Bauer’s direct testimony, 

which will emphasize that historic cell site analysis can only convey “approximate” 

location information. SA Bauer will explain that cell phones connect to the cell tower 

with which the phone has the strongest, clearest signal but that real world factors, 

such as obstacles located in the signal path, can affect signal strength and clarity so 

that the strongest, clearest signal does not always correlate to the closest town. The 

net effect of this testimony will be to make clear to the jury that historic cell site 

analysis conveys general—not pinpoint accurate—information about phone location, 

thereby curing the concerns raised by defendant. 

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Further, defendant will no doubt emphasize the approximate nature of SA 

Bauer’s analysis through cross examination. Thus, defendant’s Rule 403 concern 

about juror confusion are without merit. Given the plethora of cases establishing the 

reliability of this testimony and the government’s intention to elicit testimony 

concerning the limitations of the analysis, the court can decide this issue without a 

Daubert hearing.

CONCLUSION

For the reasons set forth above, defendant’s motion should be denied. 

Respectfully submitted,

JOHN R. LAUSCH, JR.

United States Attorney

By: s/ Michael J. Kelly

 MICHAEL J. KELLY

TIMOTHY STORINO

Assistant United States Attorneys

219 South Dearborn Street

Chicago, Illinois 60604

(312) 353-4220

michael.kelly@usdoj.gov

Date: May 28, 2021

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