A report says a smaller fire was extinguished shortly before a massive blaze at a Chevron Corp. refinery in Northern California last month.

The report given to Contra Costa County officials by Chevron on Wednesday says there was a small fire at the refinery about 10 minutes before the larger blaze.

The 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 fire was caused by a leaking 8-inch pipe but investigators also were looking into the composition of a white cloud that formed minutes before the blaze. The report cast doubt on whether the cloud consisted of flammable hydrocarbon vapor, suggesting it was mostly steam from firefighters knocking down the smaller blaze.