Explosion in Manhattan neighbourhood injured 29 people on Saturday evening
Five suspicious devices have been found near a train station in New Jersey, with one exploding during attempts to disarm it.
The blast came as five men were detained and questioned over a bomb attack in New York on Saturday night, which left 29 injured.
It has been reported the device that detonated in Manhattan, and another device found nearby, were both shrapnel-filled pressure cookers, similar to those used in the attack on the Boston Marathon in 2013.
Nearly a thousand extra police are on the streets as world leaders arrive in the city for a UN summit on the refugee crisis.
The latest device was found after the alarm was raised in the early hours of this morning in the city of Elizabeth, less than 15 miles southwest of Manhattan, after two men found a backpack containing the suspect devices in a bin.
The city's mayor, Christian Bollwage, told Sky News the pair had alerted authorities after they "saw wires and a pipe".
Up to five devices were found in the bag and when the robot cut a wire on one of them to try and disable it, it exploded.
Mr Bollwage said: "I can imagine that if all five of them went off at the same time, that the loss of life could have been enormous if there was an event going on."
FBI Bomb Squad is on scene and continuing the investigation at the train station in Midtown Elizabeth. pic.twitter.com/qvmzsgisjC— Chris Bollwage (@MayorBollwage) September 19, 2016
The discovery came after a pipe bomb exploded in a New Jersey town of Seaside Park on Saturday morning shortly before a charity run in aid of US marines and sailors.
Later that day a knife-wielding man injured eight people at a shopping mall in Minnesota, before he was shot and killed by an off-duty police officer.
As part of the inquiry into Saturday's bombing in Manhattan, the FBI pulled over a car described by officials as "a vehicle of interest in the investigation" in Brooklyn on Sunday night.
Its five occupants are being questioned at an FBI building in Lower Manhattan.
However, it has been made clear no-one has been charged with any crime and the investigation is continuing.
Forensics experts are continuing to comb through evidence from the Manhattan bombing, which was described by witnesses as a deafening blast that shattered windows and hit bystanders with shrapnel.
All of those who were injured by the explosion had been released from hospital by Sunday afternoon.
Meanwhile, CNN has been reported surveillance videos showed the same man close to the site of the first explosion and the location where the unexploded device was found.