PENTTBOM (sometimes referred to as PENTTBOMB) is the codename for the Federal Bureau of Investigation's probe into the September 11 attacks of 2001, the largest criminal inquiry in the FBI's history. Its name stands for "Pentagon/Twin Towers Bombing Investigation". The investigation was launched on September 11, 2001, and involved 4,000 special agents and 3,000 professional employees.

The FBI was able to identify the 19 hijackers within a matter of days as few suspects made any effort to conceal their names on flight, credit card, and other records.

Three of the hijackers carried copies of an identical handwritten letter (in Arabic) that was found in three separate locations: the first, in a suitcase of hijacker Mohamed Atta that did not make the connection to American Airlines Flight 11 that crashed into the North Tower of the World Trade Center; the second, in a vehicle parked at Washington Dulles International Airport that belonged to hijacker Nawaf al-Hazmi; and the third at the crash site of United Airlines Flight 93 in Shanksville, Pennsylvania.

According to the testimony before the House Intelligence Subcommittee on Terrorism and Homeland Defense on October 3, 2001, given by J. T. Caruso, the Deputy Assistant Director of the FBI's Counterterrorism Division, "translations of the letter indicate an alarming willingness to die on the part of the hijackers."

According to testimony by Susan Ginsberg, a staff member of the National Commission on Terrorist Attacks Upon the United States, in the January 26, 2004, Public Hearing:

Four of the hijackers' passports have survived in whole or in part. Two were recovered from the crash site of United Airlines flight 93 in Pennsylvania. These are the passports of Ziad Jarrah and Saeed al Ghamdi. One belonged to a hijacker on American Airlines flight 11. This is the passport of Satam al Suqami. A passerby picked it up and gave it to a NYPD detective shortly before the World Trade Center towers collapsed. A fourth passport was recovered from luggage that did not make it from a Portland flight to Boston on to the connecting flight which was American Airlines Flight 11. This is the passport of Abdulaziz al-Omari.

In addition to these four, some digital copies of the hijackers passports were recovered in post-9/11 operations. Two of the passports that have survived, those of Satam al-Suqami and Abdulaziz al-Omari, were clearly doctored. These passports were manipulated in a fraudulent manner in ways that have been associated with al Qaeda.

The passport of hijacker Satam al-Suqami was recovered a few blocks from the World Trade Center.