A police bomb squad is also analysing a stolen U-Haul van recovered from the scene.
The shooting started after two people allegedly carrying long guns drove the van to a storefront near Martin Luther King Jr Boulevard, then continued to a nearby cemetery and, finally, a supermarket where five bodies were recovered.
According to police, an officer was killed near the cemetery and suspects fled into a supermarket where the suspects fired at police as they responded to the scene.
Three people inside the supermarket were killed and another person was injured.
Another officer was shot in the shoulder and two other officers received injuries from shrapnel, according to police.
Two suspects were found dead inside the store.
The slain officer, Detective Joseph Seals, 40, was credited by his superiors with having led the department in the number of illegal guns removed from the streets in recent years, and might have been trying to stop an incident involving such weapons when he was cut down by gunfire that erupted near a cemetery, authorities said.
Jersey City police chief Michael Kelly said officers “were under fire for hours” while nearby schools were on lockdown and heavily armed and armoured Swat officers arrived on the scene, turning the town of 270,000 people across from the Hudson river in New York City into what sounded like a war zone.
Sounds of gunfire echoed through the area and were captured on mobile phones as bystanders surrounded police barricades to video the incident.
Officials were still determining what set off the gun battle, though authorities ruled out terrorism.
Police say the crime scene – which encompasses at least three locations – could take “weeks, maybe months” to investigate.
New Jersey officials shared their condolences for the people who were killed, and the White House said Donald Trump was briefed on the shootings.