Police said nothing indicated when he died but it appeared he died of a self-inflicted gunshot wound to the head from a rifle, The (Newark) Star-Ledger reported.
Bergen County Prosecutor John Molinelli identified the suspect as Richard Shoop, 20, of Teaneck, who the prosecutor said was a known drug abuser who may have had a criminal record.
No one else was injured. Molinelli said Shoop went to the mall intending to die and did not believe Shoop meant to injure anyone.
"The main motive for what he did tonight was suicide, whether self-inflicted or suicide-by-cop," Molinelli said.
Molinelli said the gunman fired at least six rounds in the mall from the .22-caliber assault-style rifle legally owned by Shoop's brother, and Shoop took the weapon without permission.
Shoop's body was found at about 3:20 a.m. EST in a typically non-public area of the mall where construction materials were being stored, The Star-Ledger said.
The two-story mall houses more than 300 storefronts.
People trapped in the mall since the shooting began shortly before closing time could be seen being escorted out in small groups, as police went store-by-store, looking for the suspect and anyone hiding from him, The Star-Ledger said.
"It took a long time to clear these stores," Molinelli said. "You've got to go foot by foot, yard by yard."