(CNN) -- The gunman who opened fire at a New Jersey mall Monday night is dead after shooting himself, a local prosecutor said.
Richard Shoop's body was found at 3:20 a.m. Tuesday in a back area of the Westfield Garden State Plaza Mall, where he fired off at least six bullets but did not strike anyone, Bergen County Prosecutor John Molinelli said.
The shooting sent panic through the shopping center shortly before closing time and set off a frenzied hunt for the gunman. In the early hours of the search, officials weren't sure whether the shooter was still inside or outside the 2-million-square-foot building.
The melee started around 9:20 p.m. ET. Witnesses said the shooter appeared calm as he worked his way through the mall with a rifle. The gunman apparently fired shots at security cameras, state police said.
Eddie Kahmann, who works inside the mall, said he heard six or seven gunshots.
"There was just people running like crazy, so I quickly just closed my doors, ran to the back, turned off all the lights, music and everything, just to stay hidden," he said.
Allie Cozic, another mall employee, said she saw "someone dressed in all black, carrying a large gun, walk past our store."
"It was just chaos. Everyone was just kind of running to wherever they could," Cozic said.
"It was almost like when you're watching a horror movie and the killer is walking slowly -- that's what it seemed like. He was wearing all black, it almost looked like body armor of some kind. As soon as I saw the gun, I just turned and ran."
Although witnesses told CNN they heard between three and nine gunshots, only one shell casing was found late Monday night, a Bergen County emergency management official said.
Despite the discrepancy in the number of gunshots heard, several witnesses reported the same oddity: a delay between shots, perhaps 15 to 20 seconds long.
A history of trouble
Shoop, 20, was well known by law enforcement. He had a history of drug use and abuse, Molinelli said.
"We believe the motive was suicide -- either self inflicted, or, God forbid, or suicide by cop," the prosecutor said.
But Molinelli said he doubts Shoop was trying lure cops into killing him because he went into a hard-to-access back area of the mall before shooting himself.
(story by Holly Yan and Dana Ford, CNN -- CNN's Susan Candiotti, Chuck Johnston, Eden Pontz, Sarah Aarthun, Scott Garber and Laura Dolan contributed to this report.)