Survivors and family members of the 168 people who died in the bombing of the Oklahoma City federal building have gathered to commemorate the 15th anniversary of the attack.
Hundreds of people attended the ceremony at the Oklahoma City National Memorial to remember those killed in the April 19, 1995, explosion. More than 600 others were injured in the attack at the Alfred P Murrah Federal Building.
Before the ceremony, bells tolled in Oklahoma City's downtown and some family members visited the site of the federal building razed in the attack, where chairs to honour the bombing victims now stand.
Oklahoma City Mayor Mick Cornett said the day of the bombing is recalled with reverence, "not because we can't forget but because we choose to remember".
"We have chosen strength, we have chosen optimism, we have chosen freedom, we have chosen to move forward together with a level of unity that is unmatched in any American city," Cornett said at the ceremony.
US Department of Homeland Security Secretary Janet Napolitano, Gov Brad Henry and former Gov Frank Keating were also scheduled to speak.
Attending the ceremony was Charlie Hangar, the Oklahoma Highway Patrol trooper who stopped bomber Timothy McVeigh on Interstate 35 the day of the bombing because his 1977 Mercury Marquis did not have a licence plate. Hangar, now the Noble County sheriff, read the memorial's mission statement before the service.
People across Oklahoma City observed 168 seconds of silence to honour the dead.
McVeigh was convicted on federal murder charges and executed in 2001. McVeigh's Army buddy, Terry Nichols, was convicted on federal and state bombing-related charges and is serving multiple life sentences at a federal prison in Colorado.