A U.S. safety agency said on Jan. 13 that a series of problems at Exxon Mobil Corp's California refinery caused an explosion there nearly a year ago, as questions mounted about when the plant, which supplies 10 percent of the state's gasoline supply, would be fixed.

An investigator for the U.S. Chemical Safety and Hazard Investigation Board (CSB), Mark Wingard, said at an interim update of the board's probe that in February 2015 a series of leaks of steam and hydrocarbons led to the blast.

Repairs are ongoing at the 149,500 barrels per day plant in Torrance, which provides 10 percent of California's gasoline supply.

Exxon had hoped to restart shut units in February , but last week Macquarie Capital said in a note that the restart would likely be pushed into the second quarter because of setbacks in repairs. That could delay finalization of the plant's pending $537.5 million sale to PBF Energy Inc , Macquarie analyst Vikas Dwivedi said in the note.

PBF announced in September it would buy the Torrance refinery, saying it would be restored to "full working order" prior to the deal's close in the second quarter of 2016.

An Exxon spokesman said on Jan. 13 that the company expects to transfer control of the refinery to PBF Energy in mid-2016; he declined comment on a timeline for the repairs. A PBF spokesman did not respond to an inquiry.

The explosion destroyed the pollution-controlling electrostatic precipitator (ESP) and damaged the plant's gasoline-making fluid catalytic cracking unit (FCCU), both of which have been shut since the blast.

Wingard said hydrocarbons leaked into the main distillation column of the FCCU, then flowed back to a reactor, out a leaking valve to the ESP, where they found an ignition source.

CSB Chairwoman Vanessa Allen Sutherland said on Jan. 13 that debris from the blast narrowly missed a tank containing hundreds of thousands of pounds of toxic hydroflouric acid, which can vaporize and spread for miles, burning eyes, skin and lungs.

"This was a serious near-miss incident," she said.

The California Division of Occupational Safety and Health in November lifted an order prohibiting Exxon from restarting the ESP and FCCU, saying they could restart when the ESP is repaired and installed.

The blast injured two workers, but caused no deaths.