RICHMOND  (AP) — Investigators probing the cause of a blaze at Chevron's Richmond refinery in Northern California are looking at heaters and responding emergency vehicles as possible ignition sources for the massive vapor cloud that spewed from a leaky pipe.

Refineries like Chevron Corp.'s have fired heaters that could have provided a spark for the Aug. 6 inferno, said Don Holmstrom, the lead investigator for the U.S. Chemical Safety Board.

"Typically in refineries there are fired heaters that have an open flame. There are a number of (possible) sources of ignition," Holmstrom said.

A responding Chevron fire truck could be another possible spark source, since the 150-to-200-foot-high vapor cloud covered a large area and engulfed more than a dozen workers who narrowly escaped serious injury, he said.

The investigator said diesel-powered trucks and generators have been ignition sources at other large refinery fires, including at BP's Texas refinery explosion that killed 15 workers and injured dozens more.

The Chevron refinery blaze knocked an important refinery unit offline, reducing the facility's production and sending thousands of people to hospitals with breathing and eye irritation complaints.

The average price for a gallon of regular on Wednesday in California was $4.09, up from $3.86 on Aug. 7.

Internal Chevron surveillance video showed the vapor cloud engulfing the area for two minutes.

The vapor cloud was estimated to be about as tall as the refinery tower.

Investigators want to examine the failed pipe blamed for the blaze, which Chevron chose not to replace nearly a year ago after an inspection.

But the charred crude unit was still too hazardous to enter on Wednesday, and the distance is still too far for them to be able to make any visual judgments about how the pipe failed, officials said.

In addition to structural steel beams melted in the fire, the area is also being compromised by at least two small hydrocarbon leaks. The leaks pose a possible health concern for investigators, and have required them to strap on respirators when entering the area.

Investigators said the small leaks were not a danger to the nearby community, just to people working in the immediate area.

The delays are worsening prospects for Chevron's ability to get the important unit back up and running. It would likely be three to four weeks before the damaged pipe can be removed for forensic testing, Daniel Horowitz, the board's managing director, said.

The conflagration destroyed an area of the refinery that produces a large amount of the gasoline that satisfies California's clean-air regulations, which are the toughest in the nation.

Other parts of the refinery, which supplies 16 percent of California's daily gas consumption, are still producing fuels.

A key target of the federal investigation is looking at why Chevron had not replaced the decades-old pipe after an inspection last fall.

Chevron examined the 8-inch line that failed and a larger, 12-inch companion line linked to it last year. But the company decided the line was good for another five years of service.

The companion line, which was the same age as the pipe that failed, had been found to be too corroded to remain in service, investigators said.

The company has refused to speculate on when the unit will be back operational but said it is cooperating fully with all of the investigations.

Contra Costa County health officials said Tuesday that more than 9,000 people went to emergency rooms for breathing problems and other health issues that might have been caused by smoke from the fire.

Many people were sickened by particulate matter in the smoke, said Wendel Brunner, director of public health for Contra Costa County.

Two or three people were hospitalized, including a girl who was admitted to Children's Hospital Oakland after reporting her asthma was made worse by smoke, officials said.