This was a day to remember, so long in coming yet so sudden.

In cemeteries where victims are buried, in firehouses where first responders heeded the alarms, and especially at the Pentagon Memorial built to commemorate that awful day, people gathered Monday to reflect on what was lost on another morning almost 10 years ago.

Tim Dudgeon sat on a bench at the Pentagon Memorial and lightly ran his fingers over the name of his fiancee, Sandra “Sandy” Taylor, engraved on one of the benches there. Taylor, 50, an Alexandria resident, was a civilian employee of the U.S. Army who died in the attacks of Sept. 11, 2001.

“It’s such a happy day,” Dudgeon said, crying as he said it. “It is one of those things, a good day but a tough day.”

Dudgeon, 63, a marketing professional from Arlington, saw the news of Osama bin Laden’s death Monday morning and was “elated.” Then his mind started to whirl. Memories came rushing back of his fiancee, a tall, outgoing blonde with, as he put it, “great legs.” The gold engagement ring with three diamonds that he bought her, miraculously recovered from the rubble. He got it back months later.

Sitting on her bench, he said, “Today I think it’s necessary I be here.”

The news that bin Laden had been shot in the head while resisting an assault by Navy SEALs on a compound in Pakistan also was a reason to look ahead. Bin Laden’s killing brought concern about possible reprisals.

A Department of Homeland Security official said the nation was at a “heightened state of vigilance.”

“Our security posture, which always includes a number of measures both seen and unseen, will continue to protect the American people from an evolving threat picture both in the next days and beyond,” the official said.

Metro is increasing security on the bus and rail systems, as are Amtrak, Virginia Railway Express and the Maryland Transit Administration. Metro spokeswoman Lisa Farbstein said transit police are working with area law enforcement agencies. She said riders will notice more uniformed officers, though it was not readily apparent during the morning commute. Amtrak said it will use police teams using bomb-sniffing dogs.

But at Arlington National Cemetery, security guards checked license plates of approaching drivers, a cautionary step “because of Osama bin Laden,” a guard said.

Area politicians simultaneously praised the assault on bin Laden’s compound and urged residents to be on the lookout for anything suspicious.

District Mayor Vincent C. Gray (D) saluted the military and intelligence officers involved in the raid, but said residents of the nation’s capital should “remain vigilant at all times.”

In Maryland, Gov. Martin O’Malley (D) called for a day of “reflection and prayer for a more peaceful future.” Bin Laden’s death, he said, “closes a sad and tragic chapter in our country and our world’s history.”

And in Virginia, the major candidates in next year’s marquee Senate race reacted quickly to the news.

Former governor Tim Kaine (D) called bin Laden’s death a “major victory in our long-fought war against terrorism,” and added that “justice was served.”

Former senator George Allen (R), who is also running for the seat being vacated by retiring Sen. James Webb (D), also said the operation brought “a measure of justice for the families who lost loved ones on September 11th.” He commended President Obama for “continuing to pursue this vile terrorist and his networks.”

And on a Twitter feed, Allen spoke with pride that the SEALs who launched the raid apparently were from Virginia, according to members of Congress: “As Virginians were hit at the Pentagon on 9-11 & USS Cole, it is appropriate that a VA-based SEAL Team brought justice directly to #Osama.”

Former Virginia tea party leader Jamie Radtke, another GOP Senate candidate, also praised Obama and the military for the victory, but warned that “we must also never forget that the evil that drove Osama bin Laden still exists — and we must be ever vigilant and strong.”

In contrast to the pure, over-the-top exuberance shown by many college-age people who gathered outside the White House on Sunday night to cheer and chant, many people tempered their satisfaction with expressions of regret, and for some, fear of reprisals.

“A demon has died,” said Air Force Lt. Col. Libby Melendez, who was traversing the Pentagon Metro station on her way to an orientation program. “It won’t replace the lives lost, but justice has been served.”

Jeff Frankson, a defense analyst from Glen Rock, N.J., said a friend’s uncle was seriously burned during the Sept. 11 attack on the World Trade Center, so the death of bin Laden has special significance.

He planned to take a few minutes today to visit the the Pentagon’s Sept. 11 memorial.

“I’m going out with a friend tonight and I will definitely have a few extra drinks,” Frankson said.“It’s a real moral victory, more than anything else.”

At first light, a handful of people gathered amid the benches at the Pentagon Memorial. ABC’s Martha Raddatz did a stand-up. A worker dragged a mat over the pebbles, evening them out with Zen-like precision. Somebody had gotten up early to leave a bouquet of red roses at the bench honoring Navy Cmdr. William H. Donovan, who died in the attacks nearly 10 years ago.

“It’s nice to be here on this day,” said Peggy Novick, 50, a hospital employee from Glocester, R.I. “It is a bit strange — it has been so long. I’m happy for the people who lost somebody and the people at the Pentagon.”

While other tourists said they were also glad to hear news of bin Laden’s death, they exhibited another emotion: fear of what could come next.

“There ain’t no words to cover it,” said Kelly Schaupert, 59, a truck driver from Missouri. “I just know what’s going to happen: something.”

“End of one era,” said his wife, Shirlene, also 59. “What’s coming next?”

Staff writers Christy Goodman, Ashley Halsey, Justin Jouvenal, Ann Marimow, Ben Pershing, Ann Scott Tyson and Nikita Stewart contributed to this report.