OKLAHOMA CITY - Several hundred people have gathered at the former site of the Oklahoma City federal building to commemorate the 20th anniversary of the terrorist bombing there that killed 168 people and injured many others.
Survivors of the April 19, 1995, attack and relatives of some of the dead are among those attending Sunday's memorial service. The service is set to begin at 9:02 a.m., which was when the bomb detonated, with a 168-second moment of silence followed by a reading of the names of those killed.
Former President Bill Clinton and Oklahoma Gov. Mary Fallin are expected to speak at the gathering.
Timothy McVeigh, an Army veteran with strong anti-government views, was executed for carrying out the bombing. His accomplice, Terry Nichols, is serving life in prison.
Officials estimate that almost 400,000 people in Oklahoma City knew someone who was killed or injured in the bombing, reports CBS affiliate KWTV in Oklahoma City.
The first time the victims were honored was on April 23, just four days after the bombing. Mourners gathered at the State Fair Park for a solemn ceremony, including Rev. Billy Graham, then-President Clinton and then-Attorney General Janet Reno.
On May 4, authorities ended the search for survivors. KWTV reports one rescue worker was killed and 85 others were injured trying to help save victims.
© 2015 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.
