Norwich — Fire officials have been unable to determine the cause of a three-alarm fire that destroyed one apartment home and damaged two other homes on Sunday.

The blaze at 48 Roath St., which left 19 people displaced from their homes and sent three firefighters to the hospital, was found to have started beneath a staircase on the outside of the house, according to Norwich Fire Department Chief Tracy Montoya. But it wasn't clear how it started, he said.

The fire started at 1:37 p.m. on the left side of the house; flames had engulfed the outdoor staircase and were spreading quickly to the second floor and attic by the time firefighters arrived at the scene.

Three firefighters were treated for heat exhaustion at Backus Hospital and released.

Montoya said Sunday afternoon's temperatures in the 90s, the close proximity of the three houses, a number of vehicles in driveways and on the street and an initial lack of personnel hindered the firefighting effort. Nine fire departments, including all six Norwich departments and crews from New London, the U.S. Naval Submarine Base and Mohegan Tribal departments responded.

The investigation into the fire was closed as of Thursday, Montoya said.