Approximately 10,000 gallons of crude oil sprayed on to the streets of Los Angeles after a high-pressure pipe burst. The oil was seen shooting up into the sky and onto streets and nearby businesses.
The Los Angeles fire department says the spill covered a half-mile area and is knee deep in some areas.
No injuries have been reported.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.