Officials investigating Sunday's fire at a Roanoke furniture store have ruled it arson.

Firefighters were called just before 6 p.m. to the Williamson Road Plaza strip mall in the 5300 block of Williamson Road Northwest, said department spokeswoman Tiffany Bradbury.

When crews arrived, they saw thick smoke coming from a storage room at Weekend Sofa Outlet.

Investigators determined the fire was intentionally set because they found evidence of accelerant use and several points of ignition, Bradbury said. She would not name the types of accelerants because of the ongoing investigation.

A damage estimate is not yet available, but the store sustained heavy fire and smoke damage, Bradbury said. Other businesses in the plaza also sustained smoke damage. Nature's Outlet was closed Monday because of the damage, Bradbury said.

Roanoke police and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives are helping the fire department investigate. The ATF is assisting because the fire involved a commercial building.

Anyone with information about the arson is asked to call the Roanoke fire marshal's office at 853-2795 or the ATF at 983-6922. The ATF is offering a $5,000 reward for information leading to an arrest and conviction.

-- Amanda Codispoti