Middlesex County Prosecutor Bruce Kaplan said the gunman, an employee who had been working at the store for about two weeks, killed two co-workers before taking his own life, The (Newark) Star-Ledger reported.
Neighbors and a law enforcement source whose name was not reported identified the shooter as Terence S. Tyler, 23, a former Marine, the newspaper said. The two victims were identified as Christina LoBrutto, 18, and Bryan Breen, 24.
Tyler had been working at the store Thursday night but left and came back armed with an AK-47 assault rifle and an automatic pistol.
Dragan Jovanovic, general manager of the Staples store next to the grocery store where the shooting occurred, told the newspaper he had learned from a Pathmark employee that Tyler threw a shopping cart through the window of the store and opened fire. Some of the victims' co-workers managed to get out of the store through a rear entrance, the report said.
Marine spokeswoman Capt. Kendra Motz told the Star-Ledger Tyler served less than two years in the Marine Corps before he was discharged Feb. 27, 2010. His specialty classification was "0311-Rifleman."
"This is the worst phone call a mayor can receive," Mayor Owen Henry told the newspaper in describing the 4 a.m. call he received notifying him of the shooting. "You can prepare for these things, but you can't prevent them."