Complex sentence: Students must pass through metal detectors and swipe ID cards to enter the buildings, due to two school shootings in East New York in 1991--1992 that, combined, resulted in the deaths of three students and the injury of one teacher.
Split into simpler sentences: Students must pass through metal detectors and swipe ID cards to enter the buildings. This arose from two school shootings in East New York in 1991--1992 that, combined, resulted in the deaths of three students and the injury of one teacher.