Flames shot out of the windows of Hazen Auto Repair & Custom and smoke billowed from both ends of the building at around 11:30 a.m.
The cause of the fire was a mechanical accident, said Lewis and Clark County Sheriff Leo Dutton. An employee was working on a turbocharger when the heat caused a fire in the cab of the vehicle, he said.
The interior of the building sustained extensive damage.
Bob Harvey, battalion chief with the Helena Fire Department, said flames were coming out of the windows when he arrived at the shop. Black smoke could be seen from blocks away.
Harvey said when firefighters arrived on scene, empoyees were running in and out of the burning building, trying to salvage what they could, removing motors and other car parts from the building at 2296 Euclid Ave. One of the employees was treated at the scene for smoke inhalation.
A crew of six firefighters initially responded and another seven were called to assist. Authorities received many 911 calls reporting the blaze, which erupted about 11:30 a.m.
Dutton said the auto body shop was renting the building. Both the repair company and the owner of the building have insurance, he added.