Al-Qaeda chief Osama bin Laden eulogized one of the September 11 hijackers in a video released on Tuesday, the sixth anniversary of the attacks that still traumatize the United States.
As Americans remembered the almost 3,000 people killed, attempted bomb attacks and security alerts kept the world on terrorist alert. A powerful bomb was disarmed in Ankara and German police launched a major operation at a US military base after receiving threats.
The Al-Qaeda leader was seen in a still image on the new video in which he gives a commentary praising hijacker Walid al-Shehri, according to IntelCenter, a US-based monitoring group that obtained an early copy of the video.
In the audio, Bin Laden describes Al-Shehri as "a young man who personally penetrated the most extreme degrees of danger and is a rarity among men: one of the 19 champions (may Allah have mercy on them all)," according to IntelCenter.
There were 19 hijackers on four planes.
Al-Shehri was aboard American Airlines Flight 11, the first jet to crash into the World Trade Center in New York on September 11, 2001.
IntelCenter said the video, which lasts 47 minutes, is titled: "The Wills of the Heroes of the Raids on New York and Washington, The Will of the Martyr (As We See Him) Abu Mus'ab Walid al-Shehri, With a Foreword by Sheikh Osama bin Laden."
In a tape released on Friday, Bin Laden mocked the United States as "weak" and threatened to escalate the war in Iraq.
News of the latest video emerged before the United States was to hold ceremonies to remember the dead from New York's Twin Towers, the US Defense Department headquarters in Washington and a hijacked plane that crashed into a field in Pennsylvania.
Also on the anniversary, Turkish police defused a powerful bomb hidden under a bus in central Ankara where a link to September 11 was immediately made.
The Turkish capital's governor, Kemal Onal, said the bomb was hidden under a bus in a car park. "The meticulous work of the police averted a possible catastrophe," he told journalists.
German police launched a major operation at the Spangdahlem US military airport after US forces received threats by telephone, police said.
The base received an anonymous call on Monday but a police spokesman said it was possible it was a hoax.
In New York, where 2,749 people were killed when two hijacked planes plowed into the World Trade Center, rescue workers were to read out the names of the dead in a solemn ceremony.
Unlike in past years, most of the ceremony will be held at a park near Ground Zero, the area where the Twin Towers once stood, and not on the site, where a memorial and other new buildings are being constructed.
In what has become an annual ritual, the reading of the names will pause for four moments of silence to mark the exact times that the planes hit the towers and when the massive buildings collapsed into piles of rubble and dust.
Church bells are to toll at 8:46 am (1246 GMT) to mark the moment that Flight 11, crashed into the North Tower.
Relatives of those killed will then be able to descend a long ramp into the World Trade Center site to lay flowers and pause momentarily.
The ceremony is a lower profile event than past commemorations. Last year, President George W. Bush laid a wreath at the site and made a televised address to the nation.
Bush, who this year called for Americans to mark the attacks with memorial services and candlelight vigils, was to attend a remembrance service in Washington and observe a moment of silence at the White House.
Near Ground Zero, New York City Mayor Michael Bloomberg was to introduce the commemorations.
"It will be a solemn occasion for remembrance and renewal, a day for us to reflect on those we lost," he said ahead of the ceremony.
In the evening, a "Tribute in Light" is to project two massive beams of light into the night sky above Ground Zero to symbolize the collapsed towers.
In Shanksville, Pennsylvania, where hijackers brought down United Airlines Flight 93 in a field after a passenger uprising, tributes were to be held to honor the 40 passengers and crew killed.
In Washington, Defense Secretary Robert Gates was to lead a memorial ceremony for the 184 were killed when American Airlines Flight 77 flew into the Pentagon.