The shooting took place on the top floor of a Stop & Shop supermarket in the West Hempstead community, while several hundred people were in the store, police said.
Gabriel DeWitt Wilson is a “person of interest” who “we believe is the shooter,” said Patrick Ryder, Nassau County Police Commissioner.
“He left the scene, and it was witnessed that he was still carrying the gun,” Ryder told reporters.
Wilson, about 30, was last seen heading west on a nearby toll road, police said. Police said Wilson is about 6 feet tall and wearing a black baseball cap and black sweatshirt.
He had worked in the store, but investigators aren’t sure he still worked there, Ryder said.
A 49-year-old shop assistant was killed in the shooting, police said. The two injured survivors were taken to a hospital and were conscious and spoke to law enforcement officials, authorities said.
Authorities say they know no motive for the shooting.
West Hempstead is located about a 10-mile drive east of the New York City borough of Queens, officials said.
Police are investigating the area and nearby schools have been shut down and have been told to secure their buildings, said Christine Geed, spokesman for Nassau County.
This story evolves. Check back regularly for updates.