NEW YORK — Ten years after the horrifying terrorist strikes, they were back at Ground Zero.

The construction workers.

The tourists.

The police officers and the firefighters.

The men and women from the financial center of the world.

On Monday, Ground Zero, once a prime al-Qaeda target, was a backdrop for a somber celebration of sorts.

“This sends a message to the bad guys, even the baddest of the bad — we’ll get ’em eventually,” said Ervin Leonard, 57, a tourist from North Carolina who said he could barely contain his enthusiasm when he first learned of Osama bin Laden’s death. “They can dance in the streets when the towers fall, but now they got to watch TV all day long and know that we got Osama.”

The scene where the massive towers had fallen was teeming with activity following the announcement that bin Laden had been killed.

Red and white construction cranes were operating, a sign that the long-awaited construction of the Freedom Tower is finally under way. Two gleaming towers have risen on the edge of Ground Zero, while a third one has begun its ascent.

On Monday, there were also reminders of what happened a decade ago: a collection of police badges and photos of fallen policemen and firemen; a glass case displaying a banged-up leather fire helmet worn by Lt. Mickey Kross, who died along with 342 other firefighters on 911.

“Ten terrible years ago a terrible evil visited this place,” Mayor Michael Bloomberg said during a visit Monday to Ground Zero. “The construction you see here is a rebuke to those who seek to destroy our freedoms and liberties. Osama bin Laden is dead and the World Trade Center site is teeming with new life. Obama bin Laden is dead and lower Manhattan is pulsating with new activities.”

The neighborhood surrounding Ground Zero filled with celebrants chanting “USA!” and “Obama Got Osama!” on Sunday night. But by Monday morning, those who had come to record the moment or to quietly pay homage to the dead had to step aside from the crush and bustle of workers and other pedestrians heading to their offices.

Maggie Landis, a 56-year-old massage therapist who watched the South Tower fall from the roof of her apartment building in Stuyvesant Town, quietly stared into Ground Zero.

“I came here because I was here on the day of the attack, and I wanted to pay my respects,” she said. “Nobody wants to celebrate the death of any individual, but this was justice. He was a murderer and he wanted to kill more innocent people.”

For others like Alexander Kochman, 18, who was only 8 at the time of the attack, it was an opportunity to see a piece of distant history up close.

“I don’t remember much, and it didn’t seem like such a big deal at the time,” he said. “But when I saw this on my laptop this morning I got so excited. It feels like it’s kind of like revenge or more like justice. He’s responsible for all this evil, and he’s not going to be sending out any more evil.”

New York City officials said that they had few illusions that the killing of bin Laden would bring an end to the threat from his network. New York City’s police commissioner, Raymond Kelly, said bin Laden and al-Qaeda had masterminded or inspired 11 terrorist attacks against New York City — most of them foiled, but two, in 1993 and 2001, successful.

For Steven Monaghan, 48, a construction worker who has spent the past decade tethered to Ground Zero — initially clearing out the rubble, and now laying down the concrete foundation to support a new generation of high rises — the news of bin Laden’s death has brought a degree of delayed justice to the families of the victims.

“Part of that dark cloud has been lifted,” he said. “Everybody seems to be walking a little more lightly.”

“It’s a bittersweet day,” Sally Regenhard, whose 28-year-old son, Christian, a firefighter, was killed at Ground Zero. “It’s good to see an evil person receive justice, but its very bitter to realize that so many good people met a brutal and needless death at the hands of this monster.”