Complex sentence: It was the deadliest shooting rampage in the United States until the 1991 Luby's massacre, and remains the deadliest shooting rampage in the United States in which the perpetrator didn't commit suicide.
Split into simpler sentences: It was the deadliest shooting rampage in the United States until the 1991 Luby's massacre. It remains, however, the deadliest shooting rampage in the United States for a mass shooting in which the perpetrator didn't commit suicide.