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

Candidate passages:
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.
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". 
Mohamed Atta varied his name on documents, also using "Mehan Atta", "Mohammad El Amir", "Muhammad Atta", "Mohamed El Sayed", "Mohamed Elsayed", "Muhammad al-Amir", "Awag Al Sayyid Atta", "Muhammad al-Amir", and "Awad Al Sayad".
In Germany, he registered his name as "Mohamed el-Amir Awad el-Sayed Atta", and went by the name, "Mohamed el-Amir" at the Technical University of Hamburg.
In his will, written in 1996, Atta gives his name as "Mohamed the son of Mohamed Elamir awad Elsayed."
When he came to the United States, he used the name "Mohamed Atta."
Atta also claimed different nationalities, sometimes Egyptian and other times told people that he was from the United Arab Emirates. 
Since they were not given boarding passes for Flight 11 in Portland, Atta and Omari checked in and went through security in Boston.
In the rushed check-in after the flight from Portland, airline officials did not load Atta's bags on Flight 11.
Suqami, Wail al-Shehri, and Waleed al-Shehri also checked in for the flight in Boston.
Wail al-Shehri and Suqami each checked one bag; Waleed al-Shehri did not check any bags.
CAPPS selected all three for a detailed luggage check.
As the CAPPS' screening was only for luggage, the three hijackers did not undergo any extra scrutiny at the passenger security checkpoint.
By 07:40, all five hijackers were aboard the flight, scheduled to depart at 07:45.
Mohamed Atta sat in business class seat 8D with Abdulaziz al-Omari in 8G and Suqami in 10B.
Waleed al-Shehri and Wail al-Shehri sat in first class seats 2B and 2A.
At 07:46, one minute behind schedule, the aircraft received clearance to push back from Gate B32, and was cleared to taxi to the runway at 07:50.
The aircraft began its takeoff run from Logan International Airport at 07:59 from runway 4R. 
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."