PORTLAND – It’s been five years since Michael Tuohey was on the receiving end of an icy stare from the ringleader of the 9/11 attacks, but the former US Airways ticket agent will be forever haunted by that brief but chilling encounter at Portland International Jetport.

Tuohey said he will never forget the anger in the eyes of Mohamed Atta after he refused to give boarding passes to him and accomplice Abdulaziz Alomari for their connecting flight in Boston, which they crashed into the World Trade Center hours later on the morning of Sept. 11, 2001.

As Monday’s anniversary of the attacks on the Twin Towers and the Pentagon approached, the reason Atta and Alomari drove to Portland rather than begin their deadly journey at Logan International Airport remains a mystery.

Michael Chitwood, who was Portland police chief at the time, said nothing has emerged to supplant the initial perception that the hijackers may have chosen to arrive separately from various locations to minimize the chance of drawing attention to themselves.

Atta and Alomari had spent the night at the Comfort Inn in South Portland and their stops that evening at a Key Bank ATM, a gas station and a Wal-Mart in the Maine Mall area were documented by video cameras. They apparently had their last meal at a Pizza Hut.

Tuohey opened the check-in station at 4:30 a.m., expecting a quiet morning with about a half-dozen flights as the post-Labor Day slowdown continued. “Everybody was in a good mood, it was a gorgeous day and things were going like clockwork,” he said.

At about 5:30 a.m., with no passengers left to check in, Tuohey was about to step away for a smoke break when he spotted two Arabic men who “looked a little confused.” The agent was surprised to see that the men had $2,400 tickets, a rarity.

Tuohey checked their baggage and went through the routine security questions, starting with Alomari and then turning to face Atta.

“I got an immediate chill in my stomach, as soon as I looked at him,” he said. “I said to myself, if this guy doesn’t look like an Arab terrorist, nobody does. He had a look on his face of contempt, and palpable anger.”

Tuohey called up the men’s boarding passes but decided to withhold their boarding cards for American Airlines Flight 11.

It was then that Atta glared at him, insisting, “They told me one-step check-in,” the Boston native, now 60 and retired, recalled in an interview at his Scarborough home.

“Now, we’re doing a hairy eyeball thing. He’s locked on me, I’m locked on him,” he said. “It seemed like a long time, but I’m sure it was just a matter of seconds.”

Finally, after Tuohey warned the two that they had to hurry to avoid missing their flight, Atta turned to Alomari, said something in Arabic and they walked away.

Tuohey said Atta, who gave him a sour look as he left, looked more menacing in person than he did on his driver’s license photo that was circulated around the world.

“In real life, the man looks even worse. All I could think of was the skeleton on a poison bottle,” the agent said.

Also on that flight was Roger Quirion of Winslow, who was traveling with fellow state worker Brian Guerette of Oakland to a conference in California.

Quirion said about 10 people were on the small commuter flight to Boston, and that Atta and Alomari were the last ones to arrive.

Unlike Tuohey, he recalls nothing sinister about Atta or his companion. “They just looked like travelers, business travelers,” Quirion said.

Upon arriving in Boston, Quirion and Guerette ended up heading off on another West Coast flight that was diverted to Cleveland. Two other West Coast-bound flights from Boston ended up crashing into the World Trade Center.

Back in Portland, Tuohey said he began to piece together what had happened after the second jetliner crashed in New York.

“It was like an ice cube in my stomach just formed immediately and every hair on my body just stood up, and I said, ‘I was right,” he said.

Tuohey, who was interviewed by the FBI as the nation’s air traffic system ground to a halt, has relived his brief encounter with the terrorists “a million times,” wondering whether there was anything he could have done to avert the attacks.

“Your rational mind is telling you one thing, but your what-if mind is pulling you in another direction,” he said

After his retirement in 2004, he started having hallucinations and pangs of guilt that caused him to become isolated and withdrawn.

“I saw Mohamad Atta at the Maine Mall. I saw him driving by my street when I was taking out the trash,” he recalled. Tuohey, who was told he had post-traumatic stress disorder, underwent counseling and things have improved.

Tuohey, who still has a yellowed copy of Atta and Alomari’s itinerary that shows their names, ticket data and flight numbers, is braced for a wave of 9/11 coverage on Monday but he plans to turn away when that day’s signature images are shown again on television.

“I can’t watch the pictures anymore. The planes hitting the building, the people jumping out, I can’t do it any more.”

As for Quirion, he said that with each passing year he has come to realize how lucky he was that fateful day.

He was on the only West Coast flight that departed Logan around 8 a.m. that fateful day that was not hijacked by terrorists. His wife, Tammy, was pregnant with their second child at the time. They now have three children.

“I just feel fortunate to be able to experience the birth of two other children and the years with my family,” he said. “I’m just lucky.

Every year, the anniversary rekindles memories for Quirion, as it does for all Americans. He said he was startled to learn how much hatred there is for America.

“It’s certainly a day that changed our world, without a doubt,” he said. “Unfortunately the results of that day are here to stay.”