Question: Why did Atta have to check two of his bags?
About the article "September 11 attacks", section "FBI".

Candidate passages:
On September 10, 2001, Atta picked up Omari from the Milner Hotel in Boston, Massachusetts, and the two drove their rented Nissan Altima to a Comfort Inn in South Portland, Maine; on the way they were seen getting gasoline at an Exxon Gas Station.
They arrived at 5:43 pm and spent the night in room 232.
While in South Portland, they were seen making two ATM withdrawals, and stopping at Wal-Mart.
FBI also reported that "two middle-eastern men" were seen in the parking lot of a Pizza Hut.
Atta and Omari arrived early the next morning, at 5:40 am, at the Portland International Jetport, where they left their rental car in the parking lot and boarded a 6:00 am Colgan Air (US Airways Express) BE-1900C flight to Boston's Logan International Airport.
In Portland, Mohamed Atta was selected by the Computer Assisted Passenger Prescreening System (CAPPS), which required his checked bags to undergo extra screening for explosives but involved no extra screening at the passenger security checkpoint.
The connection between the two flights at Logan International Airport was within Terminal B, but the two gates were not connected within security.
One must leave the secured area, go outdoors, cross a covered roadway, and enter another building before going through security once again.
There are two separate concourses in Terminal B; the south concourse is mainly used by US Airways and the north one is mostly used by American Airlines.
It had been overlooked that there would still be a security screen to pass in Boston because of this distinct detail of the terminal's arrangement.
At 6:45 am, while at the Boston airport, Atta took a call from Marwan al-Shehhi, another hijacker.
This call was apparently to confirm that the attacks were ready to begin.
Atta checked in for American Airlines Flight 11, passed through security again, and boarded the flight.
Atta was seated in business class, in seat 8D.
At 7:59 am, the plane departed from Boston, carrying 81 passengers.
The hijacking began at 8:14 am—15 minutes after the flight departed—when beverage service would be starting.
At this time, the pilots stopped responding to air traffic control, and the aircraft began deviating from the planned route.
At 8:18 am, flight attendants Betty Ong and Madeline Amy Sweeney began making phone calls to American Airlines to report what was happening.
Ong provided information about lack of communication with the cockpit, lack of access to the cockpit, and passenger injuries.
At 8:24:38 am, a voice believed to be Atta's was heard by air traffic controllers, saying: "We have some planes.
Just stay quiet and you will be OK. We are returning to the airport."
"Nobody move, everything will be OK.
If you try to make any moves you'll endanger yourself and the airplane.
Just stay quiet."
"Nobody move, please.
We are going back to the airport.
Don't try to make any stupid moves."
The plane's transponder was turned off at 8:28 am At 8:46:40 am, Atta crashed the Boeing 767 into the North Tower of the World Trade Center.
This was the first aircraft to hit the Twin Towers on the morning of September 11, 2001.
It was also the first attack of the day.
Because the flight from Portland to Boston had been delayed, his bags did not make it onto Flight 11.
Atta's bags were later recovered in Logan International Airport, and they contained airline uniforms, flight manuals, and other items.
The luggage included a copy of Atta's will, written in Arabic, as well as a list of instructions, also in Arabic, such as "make an oath to die and renew your intentions", "you should feel complete tranquility, because the time between you and your marriage in heaven is very short", and "check your weapon before you leave and long before you leave.
You must make your knife sharp and you must not discomfort your animal during the slaughter". 
 and the 9/11 Commission.
Curt Weldon's assertion that Able Danger identified the 9/11 hijackers was picked up by the national media in August 2005, after it was reported in the bimonthly "Government Security News".
In addition to asserting that Able Danger identified the 9/11 hijackers and was prevented from passing that information onto the FBI, Weldon also alleged the intelligence concerning Able Danger was provided to the 9/11 Commission and ignored.
Two 9/11 Commission members, Timothy J. Roemer and John F. Lehman, both claimed not to have received any information on Able Danger.
Following the "GSN" report, members of the 9/11 Commission began commenting on the information they had on Able Danger and Atta.
Lee H. Hamilton, former Vice Chair of the 9/11 Commission, and Al Felzenberg, a former spokesman for the 9/11 Commission, both denied that the 9/11 Commission had any information on the identification of Mohamed Atta prior to the attacks.
Hamilton told the media, "The Sept. 11 commission did not learn of any U.S. government knowledge prior to 9/11 of surveillance of Mohamed Atta or of his cell... Had we learned of it obviously it would've been a major focus of our investigation.
"On August 12, 2005, Hamilton and former 9/11 Commission chairman Thomas Kean issued a statement in response to media inquiries about the Commission's investigation of the Able Danger program.
It stated the Commission had been aware of the Able Danger program, and requested and obtained information about it from the Department of Defense, but none of the information provided had indicated the program had identified Atta or other 9/11 hijackers.
They further stated that a claim about Atta having been identified prior to the attacks had been made to the 9/11 Commission on July 12, 2004 (just days before the Commission's report was released), by a United States Navy officer employed at DOD, but thatThe interviewee had no documentary evidence and said he had only seen the document briefly some years earlier.
He could not describe what information had led to this supposed Atta identification.
Nor could the interviewee recall, when questioned, any details about how he thought a link to Atta could have been made by this DOD program in 2000 or any time before 9/11.
The Department of Defense documents had mentioned nothing about Atta, nor had anyone come forward between September 2001 and July 2004 with any similar information.
Weighing this with the information about Atta's actual activities, the negligible information available about Atta to other U.S. government agencies and the German government before 9/11, and the interviewer's assessment of the interviewee's knowledge and credibility, the Commission staff concluded that the officer's account was not sufficiently reliable to warrant revision of the report or further investigation.
Congressman Curt Weldon issued a response to the 9/11 Commission clarifying the mission of Able Danger, expressing concern over the statements made by various members of the 9/11 Commission, and promising to push forward until it is understood why the DoD was unable to pass the information uncovered by Able Danger to the FBI, and why the 9/11 Commission failed to follow up on the information they were given on Able Danger.
The 9/11 Commission has released multiple statements over the past week, each of which has significantly changed – from initially denying ever being briefed to acknowledging being briefed on both operation ABLE DANGER and Mohamed Atta.
The information was omitted primarily because they found it to be suspect despite having been briefed on it two times by two different military officers on active duty.
Additionally, the 9/11 Commission also received documents from the Department of Defense on ABLE DANGER.
Congressman Weldon reiterated these statements in testimony before the Senate Judiciary Committee on September 21, 2005. 
Two Attas theory.
Mickey Kaus of Slate.com, referring to Tom Maguire's "Two Attas" theory, speculates that "the 'Atta' fingered by Able Danger was really the first, 'Abu Nidal' Atta, and not the second, 9/11 'Al Qaeda' Atta," and that this may help explain this Able Danger issue.
Snopes.com clarified a widely circulated email that claimed the two Attas were one and the same.
Another variation of the Two Attas theory reported by Kaus notes that Omar Abdel Rahman also had an associate with the name Mohamed El-Amir (a name sometimes used by Atta) who was not the Mohamed Atta involved in the 9/11 hijacking.
However, Shaffer clarified that.
He told 9/11 Commission staffers Able Danger identified three of the individuals in the terrorist cells that conducted the 9/11 attacks, to include Atta - Shaffer did not mention the names of any other of the 9/11 hijackers in his disclosure to the 9/11 staff.
A fourth 9/11 terrorist came from the second cell.
Eric Umansky states the problem this way: "In fact, the two-Atta theory only leaves one major issue unexplained: What about the three other 9/11 hijackers that Able Danger purportedly fingered?Broeckers/Hauss described in their last German book the existence of two Attas, two Jarrahs, two Hanjours and al-Shehhis.
The Department of Defense released a report addressing the issue of two possible individuals with the last name of Atta and explaining that it was basically a clerical error.
When we reviewed INS records, they appeared to reflect two entries by Atta into the United States on January 10, 2001, which initially raised a question as to whether Atta had entered twice on the same day or whether a second person posing as Atta also entered on January 10, 2001.
The NIIS printout for the first entry reflects that Atta entered with an admission period of January 10, 2001, to September 8, 2001 (admission number 68653985708).
The second record reflects a second entry on January 10, 2001, with an admission period from January 10, 2001, to July 9, 2001 (admission number 10847166009).
However, this occurred because the inspector at the Miami District Office who changed Atta's admission date failed to follow the proper procedure to ensure that the previous entry would be corrected, and a new entry was created in NIIS.
The inspector sent the old I-94 and the corrected I-94 to the contractor which data enters I-94s for the INS.
The May 2, 2001, transaction with Atta was data entered and then uploaded to NIIS as if it were a new entry by Atta.
This happened because the inspector issued a new I-94 with a new admission number on it.
To prevent two entries from occurring in NIIS, the inspector should have crossed out the admission number on the new I-94, made a reference to the previous admission number and noted that it was not a new entry.
It should be noted that IG report is disputed by Lt. Col. Shaffer and other Able Danger team members, some of whom were never interviewed by the IG's office nor the 9/11 commission.
Congressman Weldon also claims the report was a hurried, botched up investigation that was intended to close the books on the subject rather than report on the facts.
For example this lead was never followed: "Normen Pentolino, operations manager at the Hollywood store, said two cashiers told FBI agents they might have recognized Atta, but weren't certain.
Sources inside the store said Atta may have held a BJ's membership card for more than two years."
Atta was born on September 1, 1968 in Kafr el-Sheikh, located in Egypt's Nile delta region.
His father, Mohamed el-Amir Awad el-Sayed Atta, was a lawyer, educated in both sharia and civil law.
His mother, Bouthayna Mohamed Mustapha Sheraqi, came from a wealthy farming and trading family and was also educated.
Bouthayna and Mohamed married when she was 14, via an arranged marriage.
The family had few relatives on the father's side and kept a distance from Bouthayna's family.
In-laws characterized Atta's father as "austere, strict, and private," and neighbors considered the family reclusive.
Atta was the only son, but he had two older sisters who are both well-educated and successful in their careers—one as a medical doctor and the other as a professor.
When Atta was ten, his family moved to the Cairo neighborhood of Abdeen, located near the center of the city.
Atta's father continued to keep the family private, and did not allow Atta to socialize with other neighborhood kids.
Atta spent most of his time at home studying, excelling in school.
In 1985, Atta entered Cairo University, where he studied engineering.
As one of the highest-scoring students, Atta was admitted into the very selective architecture program during his senior year.
In addition to his regular courses, Atta studied English at the American University in Cairo.
In 1990, Atta graduated with a degree in architecture, and joined the Muslim Brotherhood-affiliated Engineers Syndicate organization.
For several months after graduating, Atta worked at the Urban Development Center in Cairo, where he worked on architectural, planning, and building design.
In 1990, Atta's family moved into an 11th floor apartment in Giza. 
Immediately after the attacks, the Federal Bureau of Investigation started PENTTBOM, the largest criminal inquiry in the history of the United States.
At its height, more than half of the FBI's agents worked on the investigation and followed a half-million leads.
The FBI concluded that there was "clear and irrefutable" evidence linking al-Qaeda and bin Laden to the attacks.
The FBI was quickly able to identify the hijackers, including leader Mohamed Atta, when his luggage was discovered at Boston's Logan Airport.
Atta had been forced to check two of his three bags due to space limitations on the 19-seat commuter flight he took to Boston.
Due to a new policy instituted to prevent flight delays, the luggage failed to make it aboard American Airlines Flight 11 as planned.
The luggage contained the hijackers' names, assignments and al-Qaeda connections.
"It had all these Arab-language (sic) papers that amounted to the Rosetta stone of the investigation", said one FBI agent.
Within hours of the attacks, the FBI released the names and in many cases the personal details of the suspected pilots and hijackers.
By midday, the U.S. National Security Agency and German intelligence agencies had intercepted communications pointing to Osama bin Laden.
On September 27, 2001, the FBI released photos of the 19 hijackers, along with information about possible nationalities and aliases.
Fifteen of the men were from Saudi Arabia, two from the United Arab Emirates, one (Atta) from Egypt, and one from Lebanon.