Oklahoma City National Memorial and Museum spokesperson, Nancy Coggins says a public remembrance ceremony was held yesterday.
It began with 168 seconds of silence to remember those killed in the bombing.
A Ryder truck containing explosives blew up at 9:02 a.m. on April 19th, 1995 at the Alfred P. Murrah Federal Building in downtown Oklahoma City.
More than 300 buildings in Oklahoma City were damaged, and dozens of vehicles near the federal building were burned or destroyed.
Authorities arrested U.S. Army veterans Timothy McVeigh and Terry Nichols in connection with the crime. Investigators say both men were involved in an anti-government movement.
McVeigh was executed in June 2001, and Nichols is serving a life sentence in prison.
The Oklahoma National Memorial and Museum was dedicated at the site in 2000.
It educates visitors about events that surrounded the bombing and the impact of violence.
The Memorial was built around the Survivor Tree, which grew outside the federal building and withstood the explosion.