Here is some background information about the bombing of the Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma on April 19, 1995.
The blast killed 168 people, including 19 children. More than 500 people were injured.
The Federal Building was later razed and a park and memorial was built on the site.
The Oklahoma City National Memorial has 168 stone and glass chairs placed in rows on a lawn, one for each victim.
Both McVeigh and Nichols were former U.S. Army soldiers and were associated with the extreme right-wing and militant Patriot Movement.
The Patriot Movement rejects the legitimacy of the federal government and law enforcement.
April 19 also marks two other anniversaries. Patriots' Day is the anniversary of the American rebellion against British authority at Lexington, Massachusetts, in 1775. It is also the date that federal agents raided the compound of a religious sect in Waco, Texas, in 1993. Over 80 members of the Branch Davidian sect died in a fire that began during the raid.
McVeigh claimed he targeted the building in Oklahoma City to avenge the raid on Waco.
April 19, 1995 - At 9:02 a.m. CST, a rental truck packed with explosives is detonated outside the Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma.
April 19, 1995 - Near Perry, Oklahoma, Army veteran Timothy McVeigh is arrested during a traffic stop for driving a vehicle without a license plate.
April 21, 1995 - Timothy McVeigh is arrested by federal authorities. Co-conspirator Terry Nichols turns himself in Herington, Kansas.
May 23, 1995 - The remaining parts of the Murrah building are imploded.
August 11, 1995 - McVeigh and Nichols are indicted on murder and conspiracy charges.
April 24, 1997 - McVeigh's trial begins in Denver, Colorado.
June 2, 1997 - McVeigh is convicted on 11 counts of murder, conspiracy and using a weapon of mass destruction. He is sentenced to death on June 13.
November 2, 1997 - Nichols' trial begins.
December 23, 1997 - Nichols is convicted on federal charges of conspiracy and eight counts of involuntary manslaughter. He is sentenced to life in prison. He is serving his sentence at the Supermax federal prison in Florence, Colorado.
June 11, 2001 - McVeigh is executed by lethal injection. He is the first person executed for a federal crime in the United States since 1963.
May 26, 2004 - Nichols is found guilty in Oklahoma state court on 161 counts of murder. The jury spends five hours deliberating before announcing the verdict.
August 9, 2004 - District Judge Steven Taylor sentences Nichols to 161 consecutive life terms, without the possibility of parole.
January 20, 2006 - Michael Fortier is released from prison after serving 11 years for failing to notify authorities of McVeigh and Nichols' plans to bomb the Murrah Federal Building. Fortier was a friend of the bombers, but testified against them during their trials.