A device exploded on Monday morning as a bomb squad robot examined a suspicious package that appeared to contain pipe bombs near a train station in Elizabeth, New Jersey.
No one was hurt by the explosion.
A spokesman for Mayor Chris Bollwage originally said that the explosion was a controlled detonation, but at a press briefing later said the robot was "cutting into the device when it exploded".
He said that as many as five devices were found inside the same package, which was discovered by two men at around 8.30pm local time on Sunday.
"Based on the loudness, I think people could have been severely hurt or injured if they had been in the vicinity," he added.
The explosion follows a blast in the Chelsea district of New York just 20 miles away, and officials have said that these two incidents were not connected, although they were not ruling anything out.
Authorities are investigating whether Ahmad Rahami, who was not on US counter-terrorism lists, acted alone or as part of a terror cell.
Investigators studying CCTV footage may have identified the same man at each location, law enforcement sources reported to have said.
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