Ney York, CNN—Thousands gathered Sunday morning at New York's Ground Zero and stood still in silence, some crying as they listened to the names of victims of the September 11, 2001 attacks read aloud.
"They were our neighbors, our friends, our wives, children and parents," said New York Mayor Michael Bloomberg, who, along with President Barack Obama, helped lead the commemoration on the 10th anniversary of the attacks.
The solemn ceremony at the site of the World Trade Center came amid a heavy security presence permeating the area in lower Manhattan, as authorities continued their search for possible plotters of another terrorist strike.
Information considered credible but unconfirmed indicated up to three attackers could be trying to use an explosives-laden vehicle for an attack in New York or Washington, according to various sources. Authorities stressed that while they were taking information of a possible attack plot seriously, there was no evidence so far that an actual terrorist operation was underway.
Roads near Ground Zero were blocked and police checkpoints for both vehicles and pedestrians surrounded the memorial, with backpacks checked by bomb-sniffing dogs and put through x-ray machines.
At the ceremony, Obama and the first lady stood behind bullet-proof protection. Former President George W. Bush, who was president at the time, read a letter sent by Abraham Lincoln to a woman who lost five sons in the Civil War.
"I feel how weak and fruitless must be any word of mine which should attempt to beguile you from the grief of a loss so overwhelming," Bush read, quoting Lincoln. "But I cannot refrain from tendering you the consolation that may be found in the thanks of the republic they died to save."
Those who lost loved ones in the attacks stepped forward to read names. In all, 2,753 people died on two airplanes and on the ground when the planes slammed into the World Trade Center towers, causing their collapse. That total does not include the 10 al Qaeda hijackers on board the planes.
The New York program included music from singer James Taylor ahead of another moment of silence at 9:59 a.m., the time the South Tower collapsed. That was followed by another moment of silence at 10:03 a.m. in Shanksville, the time of the Flight 93 crash.
Those in the crowd included Joseph Conzo, who has been an EMT for 18 years. He recalled the moment the second tower collapsed, leaving him trapped beneath the smoldering rubble of a neighboring Marriott hotel.
"I still can't believe it's been 10 years," he said. "I've got a range of mixed emotions."
Conzo said he was in therapy for two years after the attacks, grappling with "survivors' guilt."
"It was the kind of thing that if you went right, you died, and if you went left, you lived," he said. "I was left with the question, 'Why me?'"
Sunday's ceremony, to him, is about closure, he said. "Let's get this finished and move forward."
A moment of silence spread across New York City at 8:46 a.m. -- the time when American Airlines flight 11 crashed into the North Tower of the World Trade Center. Houses of worship tolled their bells.
After Obama read a psalm, 167 pairs of family members began reading the names of those who perished. The reading was interrupted by another moment of silence at 9:03 a.m. -- the time when United Airlines Flight 175 struck the South Tower.
Parallel commemoration ceremonies also took place in Washington, where mourners observed a moment of silence at 9:37 a.m. -- the moment American Airlines Flight 77 struck the Pentagon, killing 184 people, and in Shanksville, Pennsylvania, where passengers aboard United Flight 93 are believed to have thwarted a plot to drive the plane through the U.S. Capitol dome and eventually caused the plane to crash in a field.
The total number of dead in New York, Washington and Pennsylvania was 2,977 people, not including 19 hijackers.
In Washington, the moment of silence was followed by remarks from Joint Chiefs of Staff Chairman Adm. Mike Mullen, Vice President Joe Biden and Secretary of Defense Leon Panetta.