Complex sentence: It was a favorite among gang members in the 1980s and early-1990s and has been used in at least two mass murder cases (The 101 California Street shootings and the Columbine High School massacre).
Split into simpler sentences: It was a favorite among gang members in the 1980s and early-1990s. It has also been used in at least two mass murder cases (The 101 California Street shootings and the Columbine High School massacre).