Two other police officers were wounded during the incident, which ended in a shootout after two gunmen barricaded themselves in a kosher supermarket.

A motive for the incident is not yet known. But the mayor said investigators believe the location was "targeted".

Several local schools and businesses were also put on temporary lockdown.

Authorities identified the police officer who died as Joseph Seals, 39, who was part of a state-wide programme to confiscate illegal weaponry.

Speaking to reporters, Jersey City Police Chief Mike Kelly said Mr Seals was a "leading police officer in removing guns from the street".

How did the attack unfold?

Mr Kelly said violence first erupted at a cemetery just after 12:00 local time (17:00 GMT) on Tuesday. Local media reported that confrontation between police and the suspects was linked to a murder investigation, but this has not been confirmed by authorities.

Police at the scene immediately came under "high-powered rifle fire", Mr Kelly told reporters, and it is believed that Mr Seals was killed while approaching the two suspects, who have not been publicly identified.

The pair fled the scene in a van and took shelter in a kosher supermarket where they held off armed police and federal officials for four hours, shooting off hundreds of rounds of ammunition.

Investigators believe the two gunmen killed three people in the shop before they were shot dead. At least one survivor from the shop is being treated for injuries.

Mr Kelly said investigators also believe the suspects' van contained an "incendiary device" which was removed and taken away for examination by police disposal specialists.

What about the motivation of the gunmen?

Officials initially said they believed the shop was chosen at random and that they did not believe it was an act of terrorism.

But on Tuesday night, Jersey City Mayor Steven Fulop said on Twitter that authorities "now believe the active shooters targeted the location they attacked".

Police said three people in the shop may have been killed by the suspects

"Due to an excess of caution the community may see additional police resources in the days/weeks ahead," Mr Fulop said, noting there has been no indication of further threats.

Officials have not elaborated on why they believe the location was targeted. They have also not commented further on whether the attack was driven by anti-Semitism.

But New York City Mayor Bill de Blasio said while there was no known threat to the city, he had also placed police on high alert, especially in Jewish communities. Jersey City is part of the New York metropolitan area.

"This tragically confirms that a growing pattern of violent anti-Semitism has now turned into a crisis for our nation," he said in a tweet. "And now this threat has reached the doorstep of New York City."