Simplify the following text for a Grade 11 native English-speaking person.
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 .