Firefighters try to put out a fire at a gatehouse building over the Little Androscoggin River, near Barker Mill Arms, in Auburn on Thursday night. Melissa Long photo

Smoke is seen from 35 Laurel Avenue in Auburn as flames engulf a building near Barker Mill Arms on Thursday evening. Max MacDonald photo

AUBURN — Police were still investigating Friday the source of a blaze that destroyed a gatehouse over the Little Androscoggin River late Thursday.

The 10:30 p.m. fire, near Barker Mill Arms, gutted the small building that housed equipment used to control flow on the river.

Auburn Fire Investigator David O’Connell on Friday said the cause was the fire was undetermined.

The building that burned was secured and surrounded by a fence when it went up in flames, O’Connell said. The building was supplied with electrical power.

All that remained of the structure Friday were some charred beams standing over a concrete slab.

The fire was first reported from Barker Mill Arms, an elderly housing complex at 143 Mill St. The gateway house is not a part of the Barker Mill Arms complex, fire officials said, and has no physical address.

One neighbor said the gatehouse sits near a woods trail that is frequently used by homeless people and others passing through the area. However, by Friday afternoon, it remained unknown if a human element was involved in the blaze.

Nobody was injured in the fire.