The various 9/11 attack and crash sites represented
the largest crime scenes in FBI history.

They were the most lethal terrorist attacks in U.S. history, taking the lives of 3,000 people, including both Americans and international citizens. The events of September 11, 2001, forever changed the FBI’s mission. Initially associated with capturing mobsters and crooked politicians, the FBI’s scope had widened considerably by the time terrorists attacked on September 11, 2001. But after 9/11, the FBI declared terrorism and "the war on terror” its top priority.

The investigation of 9/11 was the largest in the FBI's history. The search for the perpetrators began immediately with the first attack; it involved 4,000 special agents and 3,000 professional employees. More than half of the agency's work force was dedicated to identifying the hijackers and their sponsors and, along with other agencies, working to head off any possible future attacks. The FBI followed more than 500,000 leads, which included thousands of tips from the public. It was able to identify the nineteen hijackers within a matter of days, and agents quickly linked the men to the terrorist organization al Qaeda.

Although the FBI was able to identify the terrorists in a few days, the attacks highlighted some much needed changes within the organization. The 9/11 Commission found that these attacks might have been prevented if different justice agencies had been cooperating.

Since 2001, the FBI staff has hired more than 10,000 new employees. Most of these positions have been for linguists and intelligence analysts, and, in addition, the agency has increased its presence overseas. In 2005, a national security branch was created, which focuses on counter-terrorism, counterintelligence, weapons of mass destruction, and intelligence gathering.