Several other law enforcement officers were injured in the attack.
A police officer and multiple other people were killed in a furious gunbattle Tuesday that filled the streets of Jersey City with the sound of heavy gunfire for about an hour, authorities said.
Authorities said they believe the bloodshed was not an act of terrorism, but it was still under investigation.
Officials gave no immediate details on what set off the shooting and how it unfolded, and there was no word on how many suspects were involved or whether anyone had been taken into custody.
One officer was pronounced dead at a hospital, and multiple other people were found dead at a kosher supermarket, Mayor Steven Fulop said without specifying how many were killed. A second officer was struck in the shoulder by gunfire, and two others were hit by shrapnel, Fulop said.
The bullets started flying early in the afternoon in the city of about 270,000 people, situated across the Hudson River from the Statue of Liberty.
The shooting spread fear through the neighborhood, and the nearby Sacred Heart School was put on lockdown as a precaution.
SWAT teams, state police and federal agents converged on the scene, and police blocked off the area, which in addition to the school and supermarket included a hair salon and other shops. Dozens of bystanders pressed against the police barrier to capture the action on their cellphones, some whooping when bursts of fire could be heard.
Video shot by residents recorded loud volleys of gunfire reverberating along one of the city's main streets and showed a long line of law enforcement officers pointing guns as they advanced, yelling to bystanders, "Clear the street! Get out of the way!"
"It's like firecrackers going off," said Andy Patel, who works at a liquor store about three blocks away. "They were shooting like crazy. ... The cops were clearing everyone off the streets."
The shooting comes just days after another mass attack at the Naval Air Station in Pensacola, Florida, which left three sailors dead and several others injured. The gunman, who the FBI says bought his firearm legally with a hunting license, was also killed at the scene.
Officials have officially classified the investigation into the Pensacola attack as a terrorism inquiry.
This story has been updated with new information on the attack.