U.S. President Barack Obama visited Ground Zero to pay his respects and lay a wreath in remembrance of the victims of the Sept. 11, 2001, attacks, just days after Navy SEALs killed al-Qaeda leader Osama bin Laden in Pakistan.

The president closed his eyes and clasped his hands at the outdoor memorial where the twin towers of the World Trade Center once dominated the Manhattan skyline.

He shook hands with Sept. 11 family members and emergency workers at the site where the skyscrapers were brought down by planes commandeered by bin Laden's followers, killing nearly 3,000 people.

New York City Mayor Michael Bloomberg was in attendance, as was Rudolph Giuliani, who was serving as mayor when the attacks occurred in 2001.

Obama's first stop Thursday was a Midtown firehouse that lost 15 men in the attacks. The firefighters sat down with the president for a lunch they had made, which included eggplant parmesan and pasta with shrimp.

His second stop was a police station in lower Manhattan where he told officers "'there are still going to be threats out there," alluding to the death of bin Laden and the possible retribution. Of the victims of Sept. 11, he said: "We keep them in our hearts. We haven't forgotten."

Obama, who is enjoying a bump in opinion polls after bin Laden's death, said the killing sends a message to the world and the country that "when we say we will never forget, we mean what we say."  

CBC's David Common said the area around Ground Zero was already locked down early Thursday with a heavy police presence.

"It is seen as a milestone — the death of Osama bin Laden — and an opportunity for this city to move on. It has been deeply, psychologically impacted by the attacks of 9/11," Common said.

The trip also comes a day after Obama said a photo of the dead bin Laden will not be released.

"There is no doubt that we killed Osama bin Laden," he said.

Obama said he feared the release of the photos might backfire.

"It is important for us to make sure that very graphic photos of somebody who is shot in the head are not floating around as an incitement to additional violence or as a propaganda tool .… We don't trot out this stuff as trophies," he said.

Some people have expressed disbelief in the claim that bin Laden is dead, because the U.S. had not released a photograph of him. U.S. forces took bin Laden's body and buried it at sea, dropping it off the aircraft carrier USS Carl Vinson into the north Arabian Sea.

However, Obama said DNA was taken and tested, adding there is no doubt they killed the al-Qaeda leader.

"You will not see bin Laden walking on this earth again," he said.

Bush keeps low profile

Former president George W. Bush declined an invitation from Obama to attend the Ground  Zero ceremony.

Bush's decision is consistent with his desire to keep a low profile.  

"He's made the real decision not to enter into politics or the public eye," Laura Bush told The Associated Press on Thursday after appearing at a Dallas elementary school to announce grants from her foundation to school libraries.

Bush said she and her husband were at dinner Sunday night when they received word that Obama wanted to speak with him. The former president went home to take the call informing him that U.S. military forces had killed Osama bin Laden in a raid of his compound in Pakistan, she said.

He issued a statement Sunday night saying he congratulated Obama and military and intelligence personnel and called bin Laden's death "a victory for America." But his spokesman said later in the week that while the former president appreciated the offer to attend the Ground Zero event, he chooses to remain out of the spotlight in his post-presidency.