The shooting took place on a higher level at a Stop & Shop supermarket in the West Hempstead community, while a couple of hundred people were inside the store, police said.
Gabriel DeWitt Wilson is a “person of interest” who “we believe is the shooter,” said Nassau County Police Commissioner Patrick Ryder.
“He left the scene and was seen still carrying the gun,” Ryder told reporters.
Wilson, about 30, was last seen heading west on a nearby highway, police said. Police said Wilson was about 6 feet 2 inches tall and was wearing a black baseball cap and a black sweatshirt.
He had worked at the store, but investigators don’t know if he was still working there, Ryder said.
The shooting killed a 49-year-old store employee, police said. The two injured survivors were taken to a hospital and were conscious and talking to police officers, authorities said.
Authorities say they do not know the reasons for the shooting.
West Hempstead is about a 10-mile drive east of Queens County, New York City, officials said.
Police are studying the area and nearby schools are closed and have been told to secure their buildings, Nassau County spokeswoman Christine Geed said.
This story is unfolding. Please check for updates again.