A fire at a homeless encampment got away from whoever started it early Friday morning, sending smoke billowing up from under a bridge at the on-ramp to northbound Highway 101 at the Penngrove exit, authorities said.

There were no reports of injuries, though a small explosion was heard around the time of the 6:44 a.m. incident, the CHP said.

Fire crews quickly doused the fire and were in the area mopping up at 7 a.m., dispatchers said.

Some damage was noted on the underside of the bridge, though it nonetheless appeared safe, fire officials said. But firefighters were calling for Caltrans personnel to evaluate the bridge structure.