A state agency is investigating a fire that sparked inside a Chevron natural gas pipeline vault Wednesday, forcing the evacuation of about 4,000 people from 1,400 nearby homes until Thursday afternoon, authorities said. It was still unclear later Thursday whether a nearby grass fire ignited by a falling power line also caused the fire in the pipeline vault. The California Public Utilities Commission has launched a probe into what happened, said Christopher Chow, an agency spokesman. The evacuation order, issued late Wednesday night, covered an area in Bay Point between Bailey and Loftus roads and from the railroad tracks south to Hanlon Way. It was lifted about 2:30 p.m. Thursday, officials said.