From the NYTimes
"Investigators believe there were three gunmen and one of them had worked at the facility and recently had a dispute with fellow employees, according to law enforcement officials. A witness has told police that although the gunmen had their faces covered, one of them sounded and appeared very similar to an employee who had left the facility earlier in the day. “They had their appearances covered but a witness believed it had been someone who worked there,” said one official."
