On September 11, 2001, al-Omari boarded American Airlines Flight 11 and assisted in the hijacking of the plane, which was crashed into the North Tower of the World Trade Center, as part of the coordinated attacks.

[3] He attained a degree from Imam Muhammad ibn Saud Islamic University, got married, and had a daughter briefly before the attacks.

At the mosque, which experts refer to as a "terrorist factory", he was possibly taught by the radical cleric Sulayman al Alwan.

[2] According to Walid bin Attash, al-Omari was one of a group of future hijackers who provided security at Kandahar airport after their basic training at an al-Qaeda camp.

[citation needed] al-Omari eventually became involved in the planning for the September 11 attacks on the United States, an idea formulated by Osama bin Laden.

[8] al-Omari and hijacker Salem al-Hazmi entered the United States through a Dubai flight on June 29, 2001, landing in New York City.

[10] al-Omari likely stayed with several other hijackers in Paterson, New Jersey (where he rented a mailbox[11]), before moving to his own place in Vero Beach, Florida with his wife and three children.

[4][8] On his rental agreement form for that house, al-Omari gave two license-plates authorized to park in his space, one of which was registered to Mohamed Atta,[12] the attacks' mastermind.

[4] al-Omari attended the FlightSafety academy in Vero Beach with fellow hijackers Mohand al-Shehri and Saeed al-Ghamdi.

On September 6, al-Omari and fellow hijacker Satam al-Suqami flew from Florida to Boston to stay at the Park Inn Hotel.

[14] On September 10, 2001, Atta picked up al-Omari from the Park Inn Hotel, and the two drove to South Portland, Maine, in a rented Nissan Altima.

[4][15] Some sources state there is no evidence as to why they went to Portland,[4][6] whereas ABC News says it was a last-minute decision by Atta to stagger the Flight 11 hijackers' entrances into Logan International Airport on the 11th.

[14] Multiple people have claimed to see Atta and other hijackers in Portland that summer, but the FBI has found no evidence of this.

In a video recorded at the gas station, Atta has a piece of paper in his hand and shows it to al-Omari, and then they leave.

A man with the same name as those given by the FBI turned up alive in Saudi Arabia, saying that he had studied at the University of Denver and his passport was stolen there in 1995.