New Jersey Mayor Steven Fulop said authorities believe the two gunmen who killed four people, including three members of the ultra-Orthodox community before being shot dead at a kosher supermarket in Jersey City on Tuesday "targeted the location they attacked."

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Writing on Twitter, Fulop says "the community may see additional police resources in the days/weeks ahead."

Based on our initial investigation (which is ongoing) we now believe the active shooters targeted the location they attacked. Due to an excess of caution the community may see additional police resources in the days/weeks ahead. We have no indication there are any further threats — Steven Fulop (@StevenFulop) December 11, 2019

A police officer was also shot dead killed before the gunmen were killed. The two gunmen and three victims believed to be bystanders were pronounced dead inside the JC Kosher Supermarket.

Police at the scene of the shooting in New Jersey

The violence began after noon in the heart of Jersey City, the state's second-largest municipality directly across the Hudson River from Lower Manhattan, prompting a lockdown of all Jersey City schools as law enforcement swarmed the area.

The kosher supermarket targeted in the New Jersey shooting

Police arriving on the scene immediately came under "high-powered rifle fire," Jersey City Police Chief Michael Kelly told reporters.

Investigators believe the two suspects, who were not yet identified, acted alone, officials said.

"Based on what we now know, there are no ongoing threats to public safety that have any connections to today's events," Governor Phil Murphy said at a separate news conference.

"This has been an incredibly tough day for Jersey City and the entire state of New Jersey."

The slain police detective was fatally wounded near a cemetery about a mile away where the shooting apparently started when he tried to intercept one of the suspects, Kelly said.

The scene of the shooting in Jersey City

Kelly said he believed the civilians and the detective were all struck by gunfire from the suspects, who arrived in a stolen U-Haul truck and held police at bay around the store for hours before the shooting ended.

"Their movement was rapid and continuous for four hours within that area," the chief said at an news briefing. He described the crime scene as "very extensive," encompassing at least three locations. Hundreds of rounds were fired, he said.

Kelly said investigators believed the U-Haul vehicle contained what may have been an "incendiary device," which he said was removed and taken away for examination by police bomb squad personnel.

Police helping the wounded at the scene of the shooting ( Photo: AFP )

Two more police officers and a civilian were wounded but were reported in stable condition, the Hudson County Prosecutor's Office posted on Twitter. Police said the two wounded officers were later released from the hospital.

The officer who died was identified as Joe Seals, a married father of five who had been with the Jersey City force since 2006, Mayor Fulop told reporters. Kelly said Seals was in his 40s.

The scene of the shooting in New Jersey

The ensuing standoff prompted a massive law enforcement deployment with local SWAT teams and federal agents rushing to the scene, as well as officers from the neighboring New York City Police Department.

"Oh my god, it's like a movie. It's crazy. ... Nothing I've ever seen before. I'm still shaking," an unidentified resident of the area told NBC's New York affiliate.

Police at the scene of the New Jersey shooting ( Photo: AP )

Kelly said the incident occurred in an "up and coming" neighborhood of Jersey City, a racially mixed town of about 265,000 residents.