WASHINGTON – Newly released documents suggest that the mastermind of the Sept. 11, 2001, terrorist attacks flew out of Portland, rather than Boston, in order to avoid suspicion that might arise if all 10 hijackers arrived at Logan International Airport at one time.

Mohammed Atta apparently expected to avoid scrutiny by authorities in Boston by obtaining a boarding pass for American Airlines Flight 11 in Portland, according to a Washington Post report quoting declassified documents from the Sept. 11 commission.

Atta “clenched his jaw and looked as though he was about to get angry” when the airline agent in Portland refused to issue him a boarding pass for the connecting flight in Boston, the report said.

“Atta stated that he was assured he would have ‘one-step check-in,”‘ according to the report. “The agent told [Atta and fellow hijacker Abdulaziz al Omari] that they had better get going if they were to make their flight. He said that Atta looked as if he were about to say something in anger but turned to leave.”

A lingering mystery of the attacks on the World Trade Center was why Atta and al Omari drove from Boston to Portland the night before, only to board a commuter flight to Boston the next morning.

Atta and four other hijackers crashed Flight 11 into one of the twin towers. Five other hijackers left Boston at around the same time and crashed United Airlines Flight 175 into the other tower.

Last summer, members of the National Commission on Terrorist Attacks Upon the United States rejected theories that the hijackers came to Portland to avoid tight security at a larger airport.

Atta and al Omari were screened again in Boston, where they also passed through security without incident. The weapons believed to have been used, short-bladed knives, were not banned at the time under security rules in any airport.

Commission members suggested then that convenience might have been the reason the two chose to begin the journey in Portland.

“The Portland Jetport was the nearest airport to Boston with a 9-11 flight that would have arrived at Logan in time for the passengers to transfer to American Airlines Flight 11,” the report stated.

Portland Police Chief Michael Chitwood says the declassified documents validate the initial assumption of investigators that Atta and al Omari didn’t want to be seen with the entire group of hijackers.

Chitwood also said Portland police interviewed airline personnel, and he doesn’t recall anyone saying that Atta appeared angry.

“If anything,” he said, “they were late boarding. There was never any interaction that we were made aware of, not really.”