Original: The Columbine High School massacre occurred on Tuesday, April 20, 1999, at Columbine High School in Columbine, an unincorporated area of Jefferson County, Colorado, United States, near Denver and Littleton.
Simplified: The Columbine High School Massacre was a school shooting at Columbine High School in Littleton, Colorado, near Denver, Colorado on April 20, 1999.