Saturday marks the ninth anniversary of the September 11, 2001 terrorist attacks on the United States.
The country has planned a series of events to honor the nearly 3,000 people killed in the coordinated assaults by al-Qaida hijackers. The hijackers took over four commercial airliners, crashing two of them into the twin towers of the World Trade Center in New York City. The 110-story buildings collapsed, trapping and killing many employees and rescue workers.
Another plane hit the Pentagon, the U.S. military headquarters just outside Washington, while the fourth went down in Shanksville, Pennsylvania after passengers fought their hijackers. That plane is believed to have been destined for Washington.
President Barack Obama will commemorate the anniversary Saturday at a Pentagon memorial service, while Vice President Joe Biden will attend a ceremony in New York.
First lady Michelle Obama and former first lady Laura Bush are scheduled to speak at a ceremony in Shanksville.
Added to this year's anniversary is bitter controversy about plans by a Muslim group to build an Islamic cultural center and mosque near the former site of the World Trade Center.
Opponents say the proposal is disrespectful to the victims of the 2001 attacks, while supporters say the center will help bridge differences between the West and the Islamic world.
President Obama has said he supports Muslims' right to build a place of worship near the site, known as Ground Zero, but he also said he would not comment on the "wisdom" of doing so. Critics, including some Republican lawmakers, accused him of being insensitive to the families of the victims.
Work continues at Ground Zero on a museum and memorial to pay tribute to those who died in the 2001 attacks. In 2008, an outdoor memorial opened at the Pentagon. A national memorial also is being built at the Flight 93 crash site in Pennsylvania.
Some information for this report was provided by AP.