What to Know Five people were taken into custody by the FBI for questioning after a traffic stop on Verrazano Bridge in connection with Chelsea bomb

A suspicious bag was found at the Elizabeth, New Jersey, train station. A device, 1 of 5 in the bag, exploded when police tried to disarm it

Train service was shuttered between Newark Airport and Elizabeth, halting traffic on the busy North East Corridor ahead of the morning rush

One of five devices found in a bag near an Elizabeth, New Jersey, train station exploded early Monday as a bomb squad robot was trying to disarm it, and just hours later heavily armed FBI agents were seen in the city as authorities grew more concerned that there may be an active terror cell in the tri-state area.

The news of the suspicious bag at the Elizabeth station was first reported late Sunday, around the same time that the FBI said it had taken five people into custody for questioning in connection with Saturday's bombing in Manhattan's Chelsea neighborhood after a traffic stop on the Verrazano Bridge.

NBC News: Law enforcement officials say that they are concerned that there MAY be an active terror cell in the NY/NJ area w/ @jonathan4ny — Tom Winter (@Tom_Winter) September 19, 2016

"I'd say to all New Jerseyans that now is not the time to be intimidated," Gov. Christie said Monday. "We need to be on guard and we need to keep our eyes open."

Christie said all train and transit systems had been inspected after the explosion in Elizabeth.

The FBI said that the five individuals who were taken into custody are from Elizabeth, and law enforcement sources familiar with the investigation tell NBC 4 New York they are believed to be relatives or associates of 28-year-old Ahmad Khan Rahami, the suspect taken into custody after a shootout with cops in connection with the Chelsea bombings as well as a pipe bomb explosion in Seaside Park, New Jersey, over the weekend. It wasn't clear if the Elizabeth case was also connected.

There was law enforcement activity at an address on Elmora Avenue in Elizabeth at 6 a.m. Monday. Officials familiar with the investigation said that it's connected to the investigation at the Elizabeth NJ Transit station. Heavily armed FBI agents in camouflage were focusing on a residence above "First American Fried Chicken."

Five people were taken into custody and questioned by the FBI late Sunday into Monday after a traffic stop on the Verrazano Bridge. It comes just days after an explosion rocked Chelsea, injuring 29 people. Lori Bordonaro reports.

Police and the FBI had responded earlier to the Elizabeth NJ Transit station after two homeless men found a suspicious bag there around 8:30 p.m. Sunday; devices in the bag were later determined to be explosive. The bag was in a trash can next to a trestle near the station, Elizabeth Mayor Chris Bollwage said.

After the bag was discovered, all New Jersey-bound Amtrak trains were held at New York Penn Station in midtown, hampering travel on the busy North East Corridor. NJ Transit service was also suspended between Newark Liberty Airport and Elizabeth, affecting the Northeast and New Jersey Coast lines. Amtrak and NJ Transit serive had resumed with delays by 6 a.m.

One of five devices found in a suspicious bag exploded Monday morning as a bomb squad was attempting to disarm it with a robot. The resulting boom and the reaction of what appeared to be investigators was captured in this video. No one was injured.

Around 1 a.m. Monday, an explosion was heard near the train station in Elizabeth. A robot named "Jinx" was being used to dismantle the bomb at the time of the powerful blast and inadvertently detonated the device.

Mayor Bollwage said that five separate pipe bomb devices were found inside the bag and that those devices were pulled from the bag and separated so that they could be inspected. As the robot was "cutting wires" on the first device, it went off, startling everyone at the scene. No one was injured. Bollwage said it "could have hurt a lot of people."

Authorities were reassessing how to handle the other four devices after the first one exploded.

The devices in Elizabeth apparently looked similar to what detonated in Seaside Park, New Jersey, on Saturday morning ahead of a Marine race. No one was injured in the Seaside Park explosion.

Two bar patrons found the bag containing the devices in a trash can near train tracks. They looked inside and found pipes and wires. They then called authorities.

Former NYPD Commissioner Bill Bratton was on Today in New York Monday morning to talk about bombs found in Chelsea and Elizabeth, New Jersey.

Bollwage said that the men are being questioned at police headquarters but are not suspected of building or planting the device.

"We do not believe they were involved. We believe they did the right thing," Bollwage said.

Five men were being questioned by the FBI on Sunday night as a suspicious package at the Elizabeth train station in New Jersey halted traffic along the busy North East Corridor. Jonathan Dienst, Ida Siegal and Andrew Siff report.