Dustin Johnson, 38, is accused of starting the attack shortly after entering the Turtle Creek Valley Mental Health center in Homestead around 2 p.m. local time (1900 GMT), the Allegheny County Police Department said.
Police said Johnson went to the fifth floor of the facility where he stabbed the victims, while armed with two knives as well as what appeared to be a handgun but was later determined to be a BB gun.