By Colleen Jenkins and Molly O'Toole

BOSTON/WASHINGTON (Reuters) - Mariane and Richard Newton arrived for their flight at Boston's Logan International Airport on Sunday with both the trepidation and resolve felt by many fellow travelers flying exactly 10 years after the deadly hijackings of four civilian airplanes.

Two of the jets took off from Logan on September 11, 2001 and crashed into the north and south towers of the World Trade Center in New York.

"It was very much in the back of our minds," said Richard Newton, 59. But, "It's like being struck by lightning. What are the chances?"

The mood in the Boston airport on the 10th anniversary of the attacks felt much like any other day. Some people arrived extra early anticipating a security backlog and were pleasantly surprised when the long morning lines moved swiftly.

Passengers slipped off their shoes, took off their belts, spread their legs and arms for patdowns and X-ray screenings. Ten years in, the stiffer safety measures felt routine.

Flag ribbons and white carnations pinned to the lapels of airline workers hinted at the day's significance, and the suit-clad Transportation Security Administration agent taking photos of travelers weaving toward luggage screening belts signaled an increased alert for possible suspicious behavior.

Memories of the attacks were not far below the surface of order and calm.

Louvere Walker, a 33-year-old software trainer headed to California for a work trip, said she had some hesitation about flying on Sunday. She grew up in New York and recalled not being able to reach her parents or brothers there for some time after the planes hit.

"I remember that day almost like it was yesterday," Walker said. "I just think to myself, I could have been traveling that day."

Other passengers said they had no worries about heading into the sky. On a JetBlue Airways flight, some even watched historical footage of the smoking twin towers.

"I think it's fine (to fly)," said Bill Rambo, a 53-year-old project engineer headed to North Carolina to golf. "We're doing a better job with security."

'HIT REALLY CLOSE TO HOME'

At the Washington Dulles International Airport, where flags flew at half staff on Sunday, 24-year-old Sarah Mejia remembered her northern Virginia school being locked down 10 years ago when the attacks began. A plane from Dulles slammed into the Pentagon, killing 184 people.

A fourth plane took off from Newark International Airport and crashed in a Pennsylvania field. In all, nearly 3,000 people died as a result of the coordinated attacks.

Mejia now is close to marking her fourth year as a TSA security officer. She has worked every September 11, she said.

"It's somewhat an honor to be here today, to watch, to say that people are not afraid to fly, and we are here to help," Mejia said.

"Our family doctor was on the one that crashed here, at the Pentagon. It just hit really close to home."

Emotions at Dulles also wavered between fear and firmness. A worker checking bags for Southwest Airlines said a teenage girl told him she was scared to fly to Denver.

But Elizabeth Hartman, a 32-year-old headed to Los Angeles for a Monday meeting, said she was not concerned about what officials called a "credible but unconfirmed" threat of an al Qaeda plan to attack the United States again on the 10th anniversary.

"We're definitely more suspicious and wary, and that's probably a good thing," Hartman said. "But in some ways that wariness has been misdirected."

Charles Walker, 69, and his wife, Marilyn, arrived in Washington from Los Angeles, where they said airport security was more than they had ever seen.

Reports of the credible threat put Charles Walker, a Vietnam veteran and retired deputy sheriff, at a bit of unease, but he said they were still eager to visit the nation's capital to mark both the solemn anniversary and celebrate Marilyn's birthday. She turned 67 on Sunday.

"We have to get on with our lives," Marilyn Walker said. "We don't just need to be reminded every year. We need to be reminded every day."

(Editing by David Bailey)