The identity of the victim was not available but Suffolk's homicide and arson squads are heading up the investigation.
Jordan said arson investigators were on the scene, joining several other departments that worked for nearly two hours to get the fire under control after it started about 4:30 p.m. Four units in the building were damaged, police said.
Suffolk police Sgt. Jim Messina said the fire is not believed to have started at the restaurant.
Residents who live in the building said the blaze could mostly be seen burning on the building's second floor.
"I just saw thick black smoke coming through my walls and when I opened my door, it was a wall of smoke," said Eddie Hernandez, 28, who was able to flee the building with his girlfriend.