On April 19th, 1995, a bomb destroyed most of the Alfred P. Murrah Federal Building in Oklahoma City.
The blast killed 168 people, including 19 children in a day-care center.
More than 500 survivors were injured.
Timothy McVeigh and Terry Nichols were convicted for the attack.
McVeigh claimed he targeted the building as revenge for the deadly FBI raid on the Branch Davidian compound in Waco, Texas, two years to the day earlier.
He was executed in 2001.
Nichols remains in federal prison with a life sentence.
Services will be held today at the Oklahoma City National Memorial, which sits on the former site of the Murrah building.