Complex sentence: At the time, what was not known is that the supermarket event is connected to a string of recent and seemingly unconnected acts of violence and murder that have broken loose in Los Angeles perpetrated by the same supremacist group referred by the supermarket gunman.
Split into simpler sentences: At the time, what was not known is that the supermarket event is connected to a string of recent and seemingly unconnected acts of violence that have broken loose in Los Angeles. Unbeknownst to the general public, these murders are the work of the same supremacist group referred by the supermarket gunman.