Agents with the FBI pulled over a car on a highway in Brooklyn on Sunday night and were questioning the vehicle's occupants in connection with the investigation into the New York City bombing that injured 29 people, authorities said.

Agents stopped "a vehicle of interest in the investigation" at 8:45 p.m. Sunday, according to FBI spokeswoman Kelly Langmesser

No one has been charged with any crime and the investigation is continuing, Langmesser said.

But a government official and a law enforcement official who were briefed on the investigation said five people in the car were being questioned at an FBI building in lower Manhattan. The officials spoke on condition of anonymity because they were not authorized to speak about the ongoing investigation.

The bomb that rocked the bustling Manhattan neighbourhood of Chelsea contained residue of an explosive often used for target practice that can be picked up in many sporting goods stores, a federal law enforcement official said Sunday, as authorities tried to unravel who planted the device and why.

Video of the Orangetheory fitness center shows the strength of the blast in the Chelsea area of Manhattan 1:30

The discovery of Tannerite in materials recovered from the Saturday night explosion may be important as authorities probe whether the blast was connected to an unexploded pressure-cooker device found by state troopers just blocks away, as well as a pipe bomb blast in a New Jersey shore town earlier in the day.

The blast 11 hours earlier than Chelsea occurred at the site of a 5K race to benefit Marines and sailors in Seaside Park, N.J., didn't appear to be connected, though they weren't ruling anything out. The New Jersey race was cancelled and no one was injured.

Cellphones 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, who spoke to The Associated Press on condition of anonymity because the official was not authorized to comment on an ongoing investigation.

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Technicians in Quantico, Va., were said to be examining evidence from all sites.

All 29 of the injured people from the Manhattan incident were out of hospital by Sunday afternoon.

"We're going to be very careful and patient to get to the full truth here," Mayor Bill de Blasio said Sunday. "We have more work to do to be able to say what kind of motivation was behind this. Was it a political motivation? A personal motivation? What was it? We do not know that yet."

Witnesses in Manhattan described hearing a deafening blast, and surveillance video footage captured images of nearby windows being blown out by the explosion as people sprinted away from the scene.

The explosion that injured 29 people occurred on West 23rd Street, between 6th and 7th avenues. Police say a suspicious device on West 27th Street between 6th and 7th was removed by a bomb squad robot. (CBC)

"It was extremely loud, almost like thunder but louder," said Rudy Alcide, a bouncer at a nightclub near the blast.

"It's a residential area, but there are a lot of restaurants and businesses, and when the explosion happened, it sent people running in all directions. It blew out windows and sent debris flying into the streets," said CBC's Steven D'Souza, reporting from New York.

'I was so scared'

On Sunday night, investigators examined a suspicious device found in a trash can near a train station in Elizabeth, N.J.,

Elizabeth Mayor Christian Bollwage said of five devices found in a backpack has exploded while a bomb squad robot was attempting to disarm it.

Bollwage said that the device exploded shortly after 12:30 a.m. Monday. The FBI was leading the investigation and working to disarm the other four devices.

There were no reports of injuries. Bollwage said to expect more detonations.

Bollwage said two men called police and reported seeing wires and a pipe coming out of a package after finding it around 8:30 p.m. Sunday.

Danilo Gabrielli was in his apartment in Chelsea when the explosion happened 5:02

The New York 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.

"People didn't know what was going on, and that's what was scary," said Anthony Zayas, an actor who was in the Chelsea neighbourhood Saturday night when the bomb went off. "You didn't know if was coming from the subway beneath you, you didn't know if there were other bombs, you didn't know where to go."

Tannerite, which is often used in target practice to mark a shot with a cloud of smoke and small explosion, is legal to purchase and can be found in many sporting goods stores. Experts said a large amount would be required to create a blast like the one Saturday night, as well as an accelerant or other ignitor.

A view of a mangled dumpster at the site of an explosion that occurred on Saturday night in New York City. (Justin Lane/Reuters)

Police and federal spokespeople wouldn't comment on the presence of explosive material recovered at the scene.

The second device found blocks away, described by the same official as a pressure cooker with wires and a cellphone attached to it, was removed early Sunday by a bomb squad robot and New York City police blew it up in a controlled explosion Sunday evening, authorities said.

Homemade pressure cooker bombs were used in the Boston Marathon attacks in 2013 that killed three people and injured more than 260.