Updated Thursday, 3:30 p.m.
All evacuation orders due to a fire that threatened a natural gas pipeline in eastern Contra Costa County have been lifted as of 2:30 p.m., Contra Costa County Deputy Fire Chief Lewis Broschard said during an afternoon press conference.
The California Public Utilities Commission has launched a staff investigation into a fire in an underground utility vault that houses part of a natural gas pipeline run by Chevron.
The fire prompted evacuation orders for some 4,000 residents in Bay Point and Pittsburg and led to a response by about a dozen public agencies.
The incident started with a small grass fire near Suisun and Poinsettia avenues Wednesday evening that may have been caused by a downed electrical power line owned by Pacific Gas and Electric Co.
Crews who dealt with that vegetation blaze later learned that a fire in a nearby underground utility vault, owned by Chevron, had ignited. A 12-inch, high-pressure natural gas pipeline runs through the facility. That pipe's pressure was reduced from 480 psi to 25 psi in a "controlled and safe manner," Terence Carey, a county fire assistant chief, said during the afternoon press conference.
Chevron and Contra Costa County Fire personnel were able to enter the vault Thursday to ensure there was no gas leakage, Carey said. He said Chevron personnel will be on the scene for at least the next couple days to make repairs.
"I want to thank our citizens that were inconvenienced for quite a while, and we appreciate their patience and we're glad to get them back into their homes," Carey said.
Initially, Contra Costa County Fire Protection District spokesman Steve Hill said early Thursday that as firefighters were leaving the scene of the original blaze, they noticed smoke coming from the concrete vault, which is about the size of a pickup truck bed.
Carey said at a news conference later in the morning that crews were called back to the blaze about an hour after they were initially released.
"What we discovered was active fire burning in a vault," Carey said.
"It put that gas line at risk," Hill said. "Had that line ruptured or exploded, there would have been considerable damage."
County officials ordered evacuations within a half-mile of the facility, covering approximately 1,400 homes in Pittsburg and Bay Point.
Evacuation centers were set up at the Calvary Temple Church in Concord and at the parking lot at BART's Pittsburg/Bay Point station.
At around 6:20 a.m., BART announced that the shelter near its station was being moved to Los Medanos College.
Pittsburg residents Matt and Brandy Seville and their two kids, ages 3 and 1, had to leave after getting word from a law enforcement officer.
"He told me that it was time to evacuate," Brandy Seville said outside of the Los Medanos College shelter.
The Seville family was among those who had to move from the BART station to the college.
Chevron crews had been working to reduce the pressure inside the line.
Cary Wages, a project coordinator for Chevron, said the company believes a much smaller line, which monitors pressure on the larger pipeline, known as a transmitter line, may have ruptured and fueled the fire.
"What we're assuming is that it was the transmitter line that suffered the damage that ignited, just based on the volumes and the trends that we were seeing on the pipeline," Wages said.
Crews purged the line with nitrogen to make it safe for officials to investigate and allow residents back to their home.
Hill said crews were using video footage from a drone over the vault earlier today to help determine whether the site is safe.
"We're looking at the temperatures that are in there now because those tell us about the relative safety of the site. We want them much lower than they were overnight which was about 400 degrees Fahrenheit," Hill said.
Chevron spokesman Braden Reddall said the company got word of the blaze at around 8 p.m. Wednesday.
"Chevron Pipe Line Company (CPL) was notified of a fire caused by an electrical power line falling, which started a fire near our valve junction on the Northern Gas Line near Pittsburg," Reddall said in an emailed statement.
"CPL immediately shut down the line and dispatched a field team to investigate. CPL has initiated its emergency response procedures and is working with the California Office of the State Fire Marshal and local firefighters to evacuate the area as a precautionary measure," Reddall said.
Residents near the facility heard what's been described as a loud shrieking, or jet engine sound, as Chevron crews vented gas overnight to reduce pressure in the pipe, according to Hill.
He said it was unclear if an electrical power line caused the fire or if it fell during the blaze.
J.D. Guidi, a spokesman for PG&E, said the utility received a report of a grass fire and equipment failure in the area of Pullman and Crivello avenues at around 6:40 p.m. Wednesday.
That report included that PG&E wires were down in the area, Guidi said.
Another PG&E representative, Tamar Sarkissian, said that the utility believes birds may have come in contact with its power lines causing them to fall and ignite the initial grass fire.
Braden did not answer specific questions about the pipeline that's been threatened. Hill, the county fire department spokesman, said the line brings natural gas to Richmond's Chevron refinery.
The Office of the State Fire Marshal, which is part of Cal Fire, investigates accidents involving intrastate hazardous liquid pipelines. But because this incident involves a natural gas pipeline, it's not under the jurisdiction of the State Fire Marshal, according to Cal Fire spokesman Scott McLean.
Nevertheless, the agency dispatched a pipeline safety engineer to the scene "as a precaution since there is a liquid pipeline nearby," McLean said in an email. That engineer left Thursday morning and the office plans no more involvement.
Christopher Chow, a spokesman for the CPUC, said in an email that the agency is investigating the incident.
Inspectors from the Bay Area Air Quality Management District have been on the scene since early Thursday morning, assessing the situation, according to agency spokeswoman Kristine Roselius.
The incident is affecting train traffic on railroads owned by the Union Pacific Railroad and BNSF Railway.
Union Pacific service in the area has been suspended since last night, according to U.P. spokesman Jeff DeGraff.
Because the BNSF rail tracks are closed, Amtrak San Joaquins' trains will not stop at the commuter line's Antioch station, according to Olivia Irvin, an Amtrak spokeswoman.
Amtrak San Joaquins' trains headed to and from Oakland are being rerouted in Sacramento, Irvin said. That detour has led to delays, she said.
Classes have been canceled at Willow Cove Elementary School in Pittsburg.
Other agencies involved in the incident include the Contra Costa County Sheriff's Department, the county's health and hazardous materials program, Pittsburg Police, BART and Amtrak, among others.
KQED's Sukey Lewis and Julia McEvoy contributed reporting this story.
This post will be updated throughout the day.