It is most famous in 1999 because of the Columbine High School massacre at Columbine High School , which is not actually in the city of Littleton , but is in a ZIP code assigned to Littleton . Littleton became widely known in 1999 when the Columbine High School massacre occurred at nearby Columbine High School , which the news media erroneously reported as being located in the city . The school is actually located in Columbine , an unincorporated community in Jefferson County , yet in a ZIP code associated primarily with Littleton .