The cause of a smoky two-alarm fire at a high-rise office building on Summer Street housing Fidelity Investments has not yet been determined, a Boston Fire Department spokesman said Wednesday night.
But the fire at 245 Summer St. does not appear to be suspicious, Marc Sanders said.
Firefighters responded to the 12-story building near South Station at 12:14 p.m., after people in the building reported hearing several loud bangs, Sanders said at the scene Wednesday afternoon.
Fidelity employees were evacuated. But the building will be open for business on Thursday, Vincent Loporchio, a company spokesman said.
When firefighters arrived, heavy smoke was coming out of the back of the building. Crews entered the basement and knocked down flames inside an electrical vault there, Sanders said.
One firefighter was taken to a hospital to be treated for injuries that were not considered life-threatening, officials said.
Andrew Polak, 26, a subcontractor working in the basement at the time of the fire, said he heard “a couple loud bangs and then a couple more bangs. Then the power went out . . . I saw sparks around the corner.”
Polak said he wasn’t “exactly sure what was blowing up down there” but added, “whatever it was, it was serious.”
“As soon as I came up to the ground floor, I started seeing smoke in the hallways,” he said. “It was just pouring out smoke. . . . It definitely sounded mechanical; didn’t sound like an explosive.”
The cause of the fire remains under investigation.
Correspondent Andres Picon and Travis Andersen of the Globe staff contributed. John R. Ellement can be reached at email@example.com. Follow him on Twitter @jrebosglobe. Adam Sennott can be reached at firstname.lastname@example.org.