There were reports of “a large amount of ammunition” going off in the house, though the homeowner told firefighters there was no ammunition in the house.
Firefighters were also struggling with inadequate water pressure from hydrants.
Shortly after 2 p.m., amid reports of heavy fire in the rear of the house, all firefighters were ordered out of the smoke-filled structure as others prepared to tear open the roof of the sprawling yellow house on Sycamore Avenue, just east of Samara Drive.
The fire was first reported at about 1:20 p.m. as a closet fire.
Shrewsbury police have closed Sycamore Avenue between Broad Street/Route 35 and Shrewsbury Avenue, and expect it to remain closed for four to six hours.