US President Barack Obama is visiting Ground Zero, the site of the 9/11 attacks in New York, four days after US forces killed al-Qaeda head Osama Bin Laden in Pakistan.
Bin Laden was believed to be the mastermind of the 9/11 attacks in 2001.
Mr Obama laid a wreath in memory of the nearly 3,000 victims and spoke to relatives at the site.
He earlier told New York firefighters: "When we say we will never forget, we mean what we say."
The visit comes a day after the US president said graphic photographs of Bin Laden's dead body would not be made public.
The al-Qaeda leader was killed by US special forces in northern Pakistan on Monday. His body was then buried at sea from a US aircraft carrier.
The Pakistani military on Thursday admitted "shortcomings" for failing to locate Bin Laden and has said it will launch an investigation.
But it also warned it would review co-operation with the US if there were any more unilateral raids such as the one that killed Bin Laden.Moment of reflection
Mr Obama's first stop in New York was a fire station in midtown Manhattan. He told firefighters: "We are going to make sure that the perpetrators of that horrible act will see justice."
Mr Obama met families of the victims at the site of Ground Zero, where he laid a wreath in red, white and blue.
He made no public comments at the scene.
The BBC's Barbara Plett, in New York, says the Obama administration is very sensitive to accusations that the president is politicising his visit.
Thousands of people gathered at Ground Zero on Sunday night, waving flags and climbing street signs, as the news emerged that Bin Laden had been killed.
On Monday, Mr Obama said he had made it his top national security priority to find Bin Laden.
Ground Zero is now a building site, with construction scheduled for completion in 2013.
As well as several office towers, the area will also house the National September 11 Memorial & Museum, which comprises a museum, waterfalls and a park.
Mr Obama has decided not to publish photos of Bin Laden's body, saying the images could pose a national security risk.
"It is important for us to make sure that very graphic photos of somebody who was shot in the head are not floating around as an incitement to additional violence, as a propaganda tool. That's not who we are," Mr Obama said.
Our correspondent says there are mixed feelings in New York about the decision not to publish the pictures. While some want proof that it was Bin Laden who was killed, for others, the photos would reopen painful memories.
There are reports that among the information gathered from Bin Laden's compound were suggestions that al-Qaeda was considering attacking US trains on the 10th anniversary of the strikes of 11 September 2001.
According to a Homeland Security intelligence warning seen by the Associated Press, from February 2010 the group considered tampering with an unspecified railway track so that a train would fall off at a valley or bridge.
Meanwhile, White House officials have again changed their account of the raid on Bin Laden's compound.
They told respected media outlets, including the New York Times, that only one individual - a courier for Bin Laden - fired at US special forces. He was killed at the start of the raid.
The al-Qaeda leader was elsewhere in compound and unarmed when killed.
As recently as Tuesday, the president's spokesman had spoken of a "highly volatile" fire-fight, lasting throughout the 40-minute operation. And the White House had given the impression that an armed Osama Bin Laden had been killed in a shoot-out.
Critics have raised concerns about the legality of the operation, after the US revised its account to acknowledge Bin Laden was unarmed when shot dead.
But the US has said Bin Laden was a lawful military target, whose killing was "an act of national self-defence".Anger in Pakistan
In Pakistan, there are continuing recriminations over the failure to arrest or locate Bin Laden.
A senior Pakistani military official said one of Bin Laden's wives told investigators she had been living in the same room for five years, along with her husband.
On Thursday, the head of Pakistan's diplomatic service, Salman Bashir, again dismissed allegations his country's secret services had links to al-Qaeda, and said the investigation into the presence of Bin Laden in Abbottabad would reveal what failures there were.
Pakistan's army has long been seen as the most effective institution in an unstable country. However, Pakistani public opinion has been critical of the perceived violation of national sovereignty by the US raid.
Referring to the raid, Chief of Army Staff Gen Ashfaq Kayani was quoted by the Reuters news agency as saying: "Any similar action violating the sovereignty of Pakistan will warrant a review on the level of military/intelligence cooperation with the United States."
The BBC's Syed Shoaib Hasan, in Rawalpindi, said Pakistan's forces again denied all prior knowledge of the raid and Bin Laden's whereabouts.
However, he says there were some contradictions: the military claimed Pakistan contributed information leading to the capture of Bin Laden by providing details of phone calls made from Pakistan to Saudi Arabia.
The calls - from a Saudi man - concerned financial transactions. The last one was traced to Bin Laden's compound, the military said.
However, our correspondent in Rawalpindi says the military did not explain why, after this call, the compound was not raided or investigated.