A police officer and at least five other people have died in a series of “furious” gun battles in Jersey City, in the state of New Jersey.
Two other police officers were also wounded during the incident, which culminated in a shootout after gunmen barricaded themselves in a shop.
Several local schools and businesses were also put on lockdown.
A motive for the incident has not yet been established. Authorities say they do not believe it was a terror attack.
Authorities identified the police officer who died as Joseph Seals, 39, who was part of a programme to confiscate illegal weaponry in New Jersey.
Speaking to reporters, Jersey City Police Chief Mike Kelly said Mr Seals was a “leading police officer in removing guns from the street”.
Mr Kelly said an exchange of gunfire first erupted at a local cemetery just after 12:00 local time (17:00 GMT).
It is believed that Mr Seals, who was investigating a murder, was killed while approaching the suspects.
The pair fled the scene in a truck and took shelter in a nearby kosher supermarket where they continued to fire at police.
Dozens of armed law enforcement officials, including Swat teams and federal agents, were deployed to the scene, and five people were killed in the shop – two are thought to be the suspected gunmen.
Police said the suspects were armed with “high powered rifles” and fired “hundreds of rounds of ammunition” during the gun battle.
Mr Kelly said it is believed that three people in the shop were killed by the suspects, but stressed that all information was “preliminary” and an investigation would take time.
At least one survivor from the business is being treated for injuries, and the van used by the suspects is being investigated by bomb disposal specialists.