SAN JOSE — A fast-moving fire tore through an abandoned medical building in West San Jose early Wednesday in what officials suspect might be the fault of squatters living inside, according to the San Jose Fire Department.

The blaze was first reported at 2:23 a.m. at a vacant boarded-up two-story building in the 3800 block of Williams Road, San Jose fire Capt. Mike Van Elgort said.

Responding firefighters arrived and immediately saw black smoke emanating from the second floor and attic space, Van Elgort said. The intensity of the fire prompted officials to declare a second then third alarm within about a half hour, summoning a total of 65 firefighters to battle the blaze.

They knocked down the fire around 4 a.m. and deemed it under control at 5:30 a.m., Van Elgort said.

Witnesses told fire officials that they saw people fleeing the burning building as the fire broke out, but Van Elgort said a search turned up no occupants or fire victims. No firefighters were injured.

An exact origin or cause for the fire is under investigation, but Van Elgort said Fourth of July fireworks have generally been ruled out because the fire started in the interior. He added that neighbors had previously complained about people squatting in the empty building, which once housed medical and dental offices.

“There was some evidence that the structure had been breached, and boards had been removed,” Van Elgort said. “It’s an abandoned building with no power, and the fire starts inside, that indicates someone was in there.”