For the first time since taking office, President Barack Obama on Friday will lead tributes to the nearly 3,000 people killed on September 11, 2001 on the eighth anniversary of the shocking attacks.
Official and individual events are scheduled across the United States to remember the day when Americans watched in horror as four airliners hijacked by Al-Qaeda terrorists were flown into sites in New York, Virginia and Pennsylvania.
Obama will pay tribute to the victims in a speech at the Pentagon, then meet with relatives of those killed in the attacks, while Vice President Joseph Biden will attend commemorative events in New York, the White House said.
The day of tributes begins in New York, where two jet airliners slammed into the World Trade Center towers, killing 2,752 people and prompting then-president George W. Bush to declare a "war on terror."
At the site where the towers once stood relatives of those killed will join volunteers from across New York city to read the names of the victims.
The public reading, which has become an annual ritual, will be paused four times to mark the moments when American Airlines Flight 11 and United Airlines Flight 175 hit the buildings, and the moments when the two towers collapsed.
At nightfall, two beams of light will shoot skyward from the site.
Obama is scheduled to lead the tributes in Arlington, Virginia, where a third hijacked plane, American Airlines Flight 77, crashed into the Pentagon building.
The president, accompanied by Defense Secretary Robert Gates and Chairman of the Joint Chiefs of Staff Michael Mullen, is to observe a moment of silence, deliver a speech and lay a wreath.
He will then meet with relatives of the victims and tour a memorial to the 184 people killed on the ground and aboard Flight 77 when the plane crashed into the Pentagon.
The Pentagon memorial is the only major official monument to the victims of the September 11 attacks, with plans for similar sites in New York and Pennsylvania held up in part by financial and legal wrangling.
At the World Trade Center site, work is under way on the foundation of "Freedom Tower" -- part of a planned complex of five new skyscrapers, with a park and memorial in the middle.
But progress has been slow, and plans have been hampered by the financial crisis and the real estate downturn.
In Shanksville, Pennsylvania, where a fourth plane -- United Airlines Flight 93 -- crashed in an open field after passengers overwhelmed the hijackers, tributes begin Friday with a reading of victims' names.
Many believe the hijackers intended to crash the plane into the Capitol building in Washington.
Prayer services and interfaith remembrances are scheduled throughout the day, with a candlelight "peace vigil" closing out the commemorations.
Plans for a Flight 93 memorial have been hampered by controversy over the shape of the monument, and arguments over whether the government can seize private land for the site.
A final design, featuring a wall with the names of the victims, is scheduled to be completed by September 11, 2011.
On Capitol Hill, lawmakers on Wednesday unveiled a bronze plaque paying tribute to "the passengers and crew of Flight 93, whose brave sacrifice... not only saved countless lives but may have saved the US Capitol from destruction."
For many Americans, the anniversary of the attacks is a time to remember US troops serving abroad, including those sent to Iraq and Afghanistan after the "war on terror" declaration.
White House spokesman Robert Gibbs acknowledged Thursday that Obama has consciously tried to avoid using the "war on terror" phrase, but stressed that the president will remember those serving abroad on Friday, as he does on a daily basis.
"That takes up part of his day and is something that... he's thankful for and I think all of us are thankful for each and every day," he said.