Inland Regional Center, located on the 1300 block of South Waterman Street, serves residents of San Bernardino and Riverside counties who have developmental disabilities."Developmental disabilities" is PC speak for was once called "retardation and birth defects." The shooters' motive is puzzling.
This facility doesn't seem a likely target for gang warfare, nor a logical spot for terrorism. Since the Third Reich ended, militant eugenicists only exist in speculative fiction.
Some allege the shooter(s) wore body armor, apparently none have been apprehended. At this writing nobody has claimed "credit" for the shooting.