It’s hard to believe that it’s been 20 years.

On April 19, 1995, a truck-bomb explosion outside the Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma, left 168 people dead and hundreds more injured. The blast was set off by anti-government militant Timothy McVeigh, who in 2001 was executed for his crimes. His co-conspirator Terry Nichols received life in prison. Until September 11, 2001, the Oklahoma City bombing was the worst terrorist attack to take place on U.S. soil.