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