Mayor Chris Bollwage told reporters the bomb squad robot was "cutting into the device when it exploded" in Elizabeth, N.J. A spokesman for Bollwage had earlier described the blast as a controlled detonation.
The blast occurred shortly before 1 a.m. ET Monday. It was the second in New Jersey since Saturday morning and followed a bombing Saturday night injured 29 people in Manhattan.
Several law enforcement officials told NBC News that they are concerned that an active terrorism cell with multiple players could be at work in the New York-New Jersey area.
Authorities stopped a "vehicle of interest" in the New York blast at about 8:45 p.m. ET Sunday near the Verrazano-Narrows Bridge in New York, the FBI said. Five people were being questioned by the FBI early Monday, officials said.
None of the five people had been charged, and the investigation was ongoing, authorities told NBC News.
- Device explodes in Elizabeth, N.J.
- Surveillance video may show the same man at both New York locations where devices were found
- Governor orders 1,000 state troopers and National Guard soldiers to New York City
- Authorities investigating 911 call claiming responsibility
- Officials attempting to identify suspects from surveillance video
- Both NYC "bomb" and explosive device in New Jersey used flip-phones
The latest package was a backpack, and it was found by two men in a garbage can about 300 feet from the front door of a crowded pub in Elizabeth, according to Bollwage. When they saw wires and pipes, they dropped it and immediately went to police headquarters, he said.
"We do not believe those two are involved," the mayor said. "We believe they did the right thing."
Bollwage said he was "extremely concerned for the residents of the community" if "someone could just go and drop a backpack into a garbage can that has multiple explosives in it."
He added: "Based on the loudness, I think people could have been severely hurt or injured if they had been in the vicinity."
New Jersey Transit suspended service between Newark Airport and the Elizabeth station, and Amtrak suspended service along parts of the Northeast Corridor.