Standing before the president was a flower wreath meant to signify closure for families of the victims of the Sept. 11, 2001, terrorist attacks, four days after U.S. commandos killed Osama bin Laden in Pakistan. It also represented completion of a circle at the Ground Zero site, from crematorium to cemetery to construction site. But the thing about a wreath is that it’s just a ring with a hole in the middle of it — and maybe that was appropriate.

There will always be a gaping hole where the twin towers of the World Trade Center once stood. In its footprint on Thursday was a raw open plaza with half-laid stone and gravel trenches, where President Obama paid a brief tribute with a long minute of silence.

Loaders, excavators and backhoes stood motionless, and the giant cranes sat suspended in midair above the bowed head of president. An honor guard of police officers and firefighters was stiff and still in dress uniforms. Nothing moved in the unfinished memorial mall, except a few leaves on the limbs of the elegant pear tree known as the Survivor Tree, stirred by the breezes that always seem to swirl at Ground Zero.

About 60 relatives of victims attended the ceremony at the invitation of the White House. But there were holes in those ranks, too.

Some families find it too difficult to return to Ground Zero. Among them is the family of New York City Fire Chief Peter Ganci, who was one of 343 firefighters killed when the twin towers collapsed.

“You stand there and it’s a constant reminder of pain,” said Ganci’s son, Christopher, himself now a member of the city’s fire department. While he appreciated that bin Laden had met his just end, he said: “It’s bittersweet. You never want to wish, you never want to cheer for death.”

Nearly 10 years ago, on Sept. 14, 2001, another president, George W. Bush, came to Ground Zero bulky and workmanlike in a windbreaker and boots, stood on a heap of smoking debris with his arm around the shoulders of a firefighter and shouted through a bullhorn. To the armies of volunteers in the wreckage who couldn’t hear him, he blared, “I can hear you! The rest of the world hears you! And the people — the people who knocked these buildings down will hear all of us soon!”

Obama’s visit was a bookend to that event. But it was a far different affair, and a somewhat tricky one, given that the White House seemed anxious to find the right tone and to avoid the appearance of taking a “victory lap.” The president was visible to the general public only from a distance, slim and silver-headed and discreet in a dark suit and bright necktie, and he made no public remarks.

It was a good decision as far as Ganci was concerned. “Otherwise it looks like a stump speech,” he said. “Other politicians who have come and gone have made that mistake. It’s a solemn place. You go there to reflect and remember.”

The president made only a short private address to firefighters at the “Pride of Midtown” firehouse, Engine 54, Ladder 9, Battalion 9, at 48th Street and Eighth Avenue. The unit lost 15 men on 9/11, an entire shift, the heaviest casualties of any stationhouse in the city that day.

“When we say we will never forget, we mean what we say,” Obama told them, shortly before sitting down to an informal lunch in the firehouse. “Our commitment to making sure that justice is done is something that transcended politics, transcended party; it didn’t matter which administration was in, it didn’t matter who was in charge, we were going to make sure that the perpetrators of that horrible act — that they received justice.

“So it’s some comfort, I hope, to all of you to know that when those guys took those extraordinary risks going into Pakistan, that they were doing it in part because of the sacrifices that were made in the States. They were doing it in the name of your brothers that were lost.”

Likewise, in a brief ceremony at the Pentagon, Vice President Biden made no public remarks as he laid a floral wreath next to a blackened stone commemorating the crash of American Airlines Flight 77, which killed 184 people. The charred stone was rebuilt into the base of the western facade of the Pentagon to mark the site of the crash, and inscribed with the words “September 11, 2001.”

In anticipation of the presidential visit, the area around Ground Zero was a hive of commercial and tourist activity. On a sidewalk, a street vendor named Keith Wicks peddled T-shirts with a cartoonized bin Laden surrounded by American soldiers, and the slogan, “It Took Obama to Catch Osama.” “Just trying to get some groceries, make a couple bills,” Wicks said.

Shoppers skipped up the steps to Brooks Brothers, seemingly unaware that it had once served as a makeshift morgue. At a McDonald’s on Chambers Street, morning commuters and schoolkids ate McMuffins, oblivious to the fact that they were breakfasting where parched, traumatized, ash-encrusted firefighters had crawled through the doors crying, their ears bleeding and their hats smoking.

After a decade, some memories are fading. But some are indelible:

Peter Ganci, told that the towers were becoming unstable, shouting, “Everybody back!” — and then running forward because he still had men inside. Doctors working on first-responders whose boots had melted to their feet, “like marshmallows on a stick,” as one triage doctor said. The sidewalk in front of St. Luke’s Hospital, with 20 surgeons on call and a line of cots and IVs, waiting for wounded survivors who would not arrive. Nurses at the wreckage site, hanging bags of saline from broomsticks to help wash the eyes of rescuers. The strange finds of the salvage diggers: a single unbroken pane of glass; a singed pear tree that was, miraculously, still alive, which they dubbed the “Survivor Tree” and vowed to replant.

For many, the presidential visit served as an opportunity for remembrance. Soon, the site will no longer be an open gash, but a finished memorial and a museum. The deep holes will be filled with a reflecting pool and a cascading fountain, ringed with the names of the nearly 3,000 victims. Four buildings are planned, including a gleaming 103-story Freedom Tower already 60 stories high. Workers are struggling to complete the memorial in time for the 10th-anniversary ceremonies on Sept. 11, which Obama is scheduled to attend.

Ten years ago Obama was a state legislator in Chicago who watched the “nightmarish” events unfold on TV. Before Thursday, he had visited Ground Zero just once before, during the 2008 campaign.

But this week, as he laid the wreath of red roses, white carnations and blue hydrangeas, he entered a circle. It was the circle of commanders whose actions have been shaped by the terrible events of Sept. 11, 2001.

Staff writer Craig Whitlock contributed to this report.