PORTLAND – The ringleader of the Sept. 11 attacks strode through security at Portland International Jetport and boarded a commuter flight to start a day that ended in death and destruction.

What followed were dark days for the airport and for the airline industry in general. But those days are becoming a distant memory in Portland. Workers are finishing a colorful, $75 million passenger terminal, following completion of a pair of parking garage expansions totaling $60 million.

“It is a bit of a renaissance,” airport Director Paul Bradbury said as dozens of workers put on final touches.

When it opens Oct. 2, the expanded passenger terminal will have the largest geothermal system at a U.S. airport, as well as the latest explosive detection system for luggage. There’ll even be a system in place to recapture as much as 70 percent of the deicing substance sprayed on aircraft.

Near the ticketing counters, LED lights create funky designs on a wavy wall — some bling for the airport, which saw its last major expansion 16 years ago.

But for many across the nation, the airport will be remembered as the place Mohamed Atta started his day Sept. 11, 2001, taking a commuter flight to Boston with co-conspirator Abdulaziz Alomari. From there, they boarded American Airlines Flight 11, which Atta flew into the World Trade Center’s north tower.

Nearly 3,000 people died that day.

Among the Maine victims were retirees Jackie and Robert Norton of Lubec, who were aboard Flight 11; Portland lawyer James Roux, a passenger on United Airlines Flight 175, which hit the south tower; Stephen Ward, a Gorham native working on the 101st Floor of the north tower; and Navy Cmdr. Robert Allan Schlegel, a Maine native who died at the Pentagon.

Michael Tuohey, the ticket agent who printed out boarding passes for Atta and Alomari, recalled that Alomari smiled and presented his ID. There was no smile from Atta, however.

“He just emanated contempt and anger,” said Tuohey, who retired in 2004. “He was sallow with deep, dark brooding eyes, and he had a scowl on his face. He was obviously in a bad mood. He looked at people as if they were nothing. I guess that’s what they were. He knew he was going to die. So why should he care?”

Michael Chitwood, police chief at the time, was vacationing at the New Jersey shore on Sept. 11 and rushed back to Portland to deal with a rash of reports of terrorist sightings — and near-panic over the thought that a terrorist cell could have been operating out of Maine’s largest city.

“People were terrified. They were seeing terrorists at every corner,” said Chitwood, now police superintendent in Upper Darby, Pa. “That’s what we were dealing with — hysteria.”

In the end, there are only theories as to why Atta chose Portland, and there’s no evidence that he came to the city before the eve of Sept. 11.

Chitwood buys into the theory that Atta came to Portland to ensure he wasn’t being followed. Others think Atta wanted the group to spread out to avoid suspicion.

“Ten years later, nobody has been able to determine truly why they came and used the Portland jetport as the gateway to their scheme,” Chitwood said.

Jeffrey Monroe, Portland’s former transportation director, said screeners followed proper protocols then back then, but security measures have come a long way since.

Bradbury recalls the difficult days after 9/11 at the airport. He also recalls how people came together and refused to let the terrorists win.

“It was extremely dark for aviation,” Bradbury said. “I guess on the brilliant side, there was a spirit that we’re going to overcome this. And the patriotism was unbelievable. And the desire not to let the terrorists win was pretty amazing.”