IGNITION SOURCE: 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 huge vapor cloud that spewed from a leaky pipe.

PAST CULPRITS: 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.

UNIT DOWN: The Chevron refinery blaze knocked a refinery unit offline, reducing the facility's production and leading to higher gasoline prices in California. Thousands of people also went to hospitals with breathing and eye irritation complaints.