Authorities have questioned several people as they examine whether there is a connection between an explosion that rocked a crowded New York City neighbourhood, an unexploded pressure-cooker device found streets away, an earlier pipe bomb blast in New Jersey or explosive devices found across the state near a train station.
On Sunday night, FBI agents stopped "a vehicle of interest in the investigation" of the Manhattan explosion, according to FBI spokeswoman Kelly Langmesser.
She would not provide further details but a government official and a law enforcement source said that five people in the car were being questioned at an FBI building in lower Manhattan.
No one has been charged with any crime and the investigation is continuing, Ms Langmesser said.
Governor Andrew Cuomo, touring the site of Saturday's blast that injured 29 people in Manhattan's Chelsea neighbourhood, said there did not appear to be any link to international terrorism. He said the second device appeared "similar in design" to the first, but did not provide details.
On Sunday, a federal law enforcement official said the Chelsea bomb contained a residue of Tannerite, an explosive often used for target practice that can be bought in many sporting goods shops. The discovery of Tannerite may be important as authorities probe whether the three incidents are connected.
Mobile phones were discovered at the site of both bombings, but no Tannerite residue was identified in the New Jersey bomb remnants, in which a black powder was detected, said the official.
Authorities said the Manhattan bombing and New Jersey pipe bomb did not appear to be connected, though they were not ruling anything out. The New Jersey race was cancelled and no-one was injured.
Late on Sunday, more suspicious devices were found near a train station in Elizabeth, New Jersey.
Elizabeth mayor Christian Bollwage said the devices were found in a bag in a rubbish bin by two men who reported seeing wires and a pipe coming out of the package.
There turned out to be five devices in the bag. One of the devices exploded as a bomb squad used a robot to try to disarm it. No injuries were reported.
There was no immediate word on whether the devices were similar to those in nearby Seaside Park or New York City.
Officials have not revealed any details about the make-up of the pressure-cooker device, except to say it had wires and a mobile phone attached to it. On Sunday night, police blew up the device, rendering it safe. A forensic examination of the device will be sent to the FBI Laboratory at Quantico, Virginia, police said.
Homemade pressure cooker bombs were used in the Boston Marathon attacks in 2013 that killed three people and injured more than 260.
On Sunday, a team of five FBI agents searched an Uber driver's vehicle which had been damaged in the Manhattan blast, ripping off the door panels inside as they examined it for evidence. The driver, MD Alam, of Brooklyn, had just picked up three passengers and was driving along 23rd Street when the explosion occurred, shattering the car's windows and leaving gaping holes in the rear passenger-side door.
The Chelsea explosion left many rattled in a city that had marked the 15th anniversary of the 9/11 terrorist attacks only a week earlier and where a United Nations meeting to address the refugee crisis in Syria was scheduled on Monday.
Witnesses described a deafening blast that shattered shop windows and injured bystanders with shrapnel in the mostly residential neighbourhood on the city's west side.
As authorities tried to unravel who planted the device and why, one New Yorker, Anthony Stanhope, 40, knew exactly what had just happened.
"I was sitting in my apartment, and all of a sudden I heard a big boom, and I thought to myself wait a minute, it can't be thundering and lightning at this hour, and then all of a sudden car horns went off, and I thought oh my god this isn't lightning, this is too loud - this is a bomb," said Mr Stanhope.