NEW YORK - Americans stood in silence and world leaders paid their respects on Monday five years after hijackers crashed airliners into Washington and New York icons of US finance and government in the deadliest attack in US history.
From the urban landscape of Manhattan to the Pentagon and a rural Pennsylvania field, tearful ceremonies remembered the 3000 victims and revived traumatic memories of the day. Politicians laid wreaths and bagpipes played mournful tunes.
"It's hard to believe that it's been five years. It's always going to seem like yesterday," said Felicia Cappo, who lost a brother in the World Trade Center's south tower.
President George W. Bush and his wife, Laura, stood at New York's Fort Pitt firehouse and bowed their heads for two moments of silence, first at 8.46am EDT (12.46am NZT), the moment a plane flew into the north tower, and again at 9.03am EDT (1.03am NZT), when the south tower was hit.
They proceeded to Shanksville, Pennsylvania, for a wreath-laying ceremony on the placid countryside where hijacked United Flight 93 slammed into the ground after a passenger revolt stopped the plane from attacking Washington. Forty passengers and crew were killed.
Jarring images of the day - smoke billowing from the towers, New Yorkers crying in the streets, debris falling from the darkened sky - dominated US television and newspapers.
At Ground Zero, where the 110-story twin towers pancaked to the ground, New York police and firefighters marched down a ramp into the pit for a flag-waving ceremony on a day of crisp, clear blue skies, eerily similar to September 11 five years ago.
Spouses and partners of victims read out the names of all 2749 people who died at the World Trade Centre.
"This is where my sister was buried. The blue skies this morning are a reminder of that day. Ironically, every year seems to be the same sunny day," said Eleni Kousoulis, 36, whose younger sister, Danielle, 29, died in the north tower.
The anniversary sharpened an election-year debate over whether America, caught in a vicious unpopular conflict in Iraq, is any safer.
"We took our eye off the ball," New York Democratic Sen. Hillary Clinton told CBS' "Early Show." "I mean, we diverted resources and attention to Iraq and we didn't finish the job."
Facing the prospect that Democrats could wrest control of the US Congress from his fellow Republicans in the November election, Bush has been pushing his national security credentials as he did during his 2004 re-election campaign.
Bush, whose approval ratings have been weighed down by the Iraq war, was to attend a Pentagon ceremony and address the country from the Oval Office at 9pm EDT (1pm NZT).
At the Pentagon, where 184 people were killed on September 11, US Vice President Dick Cheney and Defence Secretary Donald Rumsfeld, both lightning rods of criticism over the Iraq war, stood as bagpipes played "Amazing Grace."
Cheney called September 11 "a day of national unity," then added later: "We have no intention of ignoring or appeasing history's latest gang of fanatics trying to murder their way to power."
His comment seemed aimed at critics of US war policies, coming two weeks after Rumsfeld angered Democrats by saying "some seem not to have learnt history's lessons" and that some politicians had wanted to appease Hitler's Germany before World War Two.
At the United Nations, UN Secretary-General Kofi Annan said the September 11 attacks inflicted a "gaping wound" on New York and reminded the world that terrorism was unacceptable, no matter who commits it.
Prime Minister Tony Blair, travelling in the Middle East, expressed his "condolences and sympathy to the families of all those who lost loved ones in that terrible attack."
Security officials were taking extra precautions. A United Airlines flight was diverted to Dallas after unclaimed objects were found aboard. It was searched and cleared.
Al Qaeda warned in a video aired on the anniversary that US allies Israel and the Gulf Arab states would be its next target. The Islamic group's No. 2, Ayman al-Zawahri, urged Muslims to step up attacks against the United States and the West, according to excerpts aired by CNN.
Osama bin Laden was nowhere to be seen. The Washington Post reported on Sunday the search for him had gone "stone cold."