Five Optimum vans burned at the company's Port Jefferson Station location as two fire departments fought to contain the flames late Wednesday night, authorities said.
The fires were reported just before 10 p.m. on Industrial Road, Suffolk County police said.
They were largely extinguished shortly before 11 p.m. as firefighters from Terryville and Port Jefferson hosed down the last van, according to the Suffolk County Department of Fire, Rescue and Emergency Services.
Authorities said there were no reports of injuries.
Other details were not available. Arson detectives are working on the investigation, which was ongoing Thursday, police said.