The device was unintentionally detonated by a bomb squad robot

The backpack was found in a garbage can in a nearby restaurant

FBI agents are reviewing surveillance footage

After a suspension, transit service has resumed

Explosion follows 2 other nearby weekend incidents

FBI questioning 5 people about N.Y. explosion Saturday

And Elizabeth man named as suspect in N.Y. case taken into custody

ELIZABETH -- An improvised explosive device in a backpack detonated near the Elizabeth train station early Monday as authorities were using a bomb robot to examine the item, officials said.

The blast occurred around 12:40 a.m. near Morris Avenue and Julian Place. The explosion was not a controlled blast, but happened unintentionally as the robot was cutting the device, according to Mayor Christian Bollwage. No one was injured in the blast, Bollwage said.

Authorities found five devices inside a single backpack near the train station, including one that was detonated. After the explosion, press and bystanders were moved back because the four other explosive devices inside of the backpack were still believed to be live, Bollwage said.

No other explosions were heard as of 5 a.m.

Authorities were using a robot to investigate a suspicious backpack underneath the train tracks at the Elizabeth train station when a device inside the backpack exploded. (Jessica Remo / NJ Advance Media for NJ.com)

On Twitter, the FBI's Newark division described the devices as "multiple improvised explosive devices."

Earlier Sunday night, the FBI detained five people in Brooklyn for questioning related to the Saturday night explosion in Manhattan that injured more than two dozen people.

The Elizabeth incident unfolded after two men found the backpack in a waste basket on North Broad Street and Julian Place around 8 p.m. Sunday, the mayor said.

The men took the backpack "because they thought it was of some value," walked for a bit, then saw wires and a pipe, dropped the package and notified Elizabeth police, he said.

Bollwage told reporters the explosives were originally found in the trash can about 300 feet from the door of Willy's Place near the train station in the city of about 125,000, which is also the Union County seat.

"If that pub was crowded and there was a lot of people there, it could have severely injured, killed and maimed many, many people," Bollwage told reporters.

Bollwage said the Elizabeth devices did not contain a cell phone or any other electronic detonators.

After the items were found, Union County's Bomb Squad was called in and used a drone to examine the backpack, the mayor said.

"The drone indicated it could be suspicious and it could be a live bomb," Bollwage said.

When asked if he ever thought Elizabeth would be the target of such an attack, Bollwage said he was not sure the city was the intended target and that the devices could have instead been dumped by a person who realized he or she was being investigated.

"I'm not willing to admit Elizabeth is a terror target," Bollwage said. "Because of the location, it's very possible someone was trying to get rid of package as opposed to set it off."

Bollwage said FBI agents are reviewing surveillance footage from the restaurant near the trash can where the backpack was first found by the two men.

4 A.M.: Police and media remain near the Elizabeth train station, where five explosive devices were found in a backpack. One of the devices detonated around 12:40 a.m. as a bomb robot was examining it. No other explosions have happened since. 9/19/16 (Jessica Remo / NJ Advance Media for NJ.com)

Reached after midnight on Monday morning, a spokesman for the FBI in Newark, Special Agent Michael Whitaker, said only that his agency had responded to the scene, and declined to provide any details of the investigation.

The FBI asked anyone with information to call 1-800-CALL-FBI.

Police cordoned off the station and many streets surrounding it. The investigation halted train service on the busy Northeast Corridor and North Jersey Coastline commuter rail lines. Later, officials said service would resume at 5:30 Monday, with delays expected.

Tickets and passes on both rail lines are being cross-honored on PATH along with NJ Transit and private buses, according to the transit agency.

Amtrak service was also suspended near Elizabeth, spokesman Craig Schulz said in a statement.

"We apologize to our customers for the inconvenience and will resume service as soon as it is safe to do so," the spokesman said.

Approximately 2,400 Amtrak passengers were impacted by the service suspension, the spokesman said.

"Robust security measures are in place at stations, on trains and along the tracks and Amtrak Police remain in close contact with local, state and federal partner agencies to coordinate and share intelligence information," the statement said. "At this time there are no specific or credible threats against Amtrak."

It was not immediately clear if the Elizabeth incident was linked to a bombing Saturday night in New York City that injured 29 people or another pipe-bomb style device that went off near a military charity race in Seaside Park also on Saturday.

BREAKING PHOTO: blast near Elizabeth NJ train station as bomb techs try to disarm device. Video soon. @PIX11News pic.twitter.com/doX6PgktXw — Anthony DiLorenzo (@ADiLorenzoTV) September 19, 2016

Bomb techs from the FBI, Union County, & the New Jersey State Police have arrived on the scene and are now rendering the area safe — FBI Newark (@FBINewark) September 19, 2016

This post will be updated as more information becomes available.

Steve Strunsky and Tom Haydon contributed to this report.

Jessica Remo may be reached at jremo@njadvancemedia.com. Follow her on Twitter @JessicaRemoNJ. Find NJ.com on Facebook.

Noah Cohen may be reached at ncohen@njadvancemedia.com. Follow him on Twitter @noahyc. Find NJ.com on Facebook.