California investigates nine-mile oil slick
Clean-up crews are working around the clock as investigators look into how tens of thousands of gallons of oil spewed into the sea off Santa Barbara.
More than 7,000 gallons (31,800 litres) of oil have been mopped from the spill - a fraction of the 20,000 gallons officials say spilled into the sea after a pipe burst on Tuesday.
Federal officials are to excavate the pipe to find clues to how it ruptured.
The company responsible for the pipeline has publicly apologised.
Up to 105,000 gallons spilled over a period of three hours on Tuesday - the majority of the oil remained on land.
More than 300 federal, state and local responders are now on the scene.
California Governor Jerry Brown declared a state of emergency on Wednesday to help the state "quickly mobilise all available resources".
"We will do everything necessary to protect California's coastline," he added.
State and local prosecutors are investigating the incident to see if criminal charges could be brought or if there are grounds for civil liability.
It is estimated about 21,000 gallons of oil have reached the ocean, forming a nine-mile (15km) slick. and it remains unclear how long the clean-up process will take or what it will cost.
"We deeply, deeply regret that this incident has occurred at all," Plains All American Pipeline LP Chairman and CEO Greg Armstrong said, apologising.
Mr Armstrong vowed that his company "will remain here until everything has been restored to normal".
Emergency workers have been fanning out across Refugio State Beach to remove oil that is stuck to sand and rocks.
Officials are pleading with members of the public that wish to volunteer to go through official channels, because of the health risks that working around the oil can present.
The 24-inch pipe was built in 1991 and had been tested a few weeks ago, company officials said.
Environmentalists are concerned that the oil may harm wildlife including birds and whales, with seals, sea lions and whales also migrating north through the area.
"It smells like what they use to pave the roads," tourist Fan Yang told the Associated Press news agency. "I'm sad for the birds, if they lose their habitat."
A wildlife official said that five oiled pelicans are now being treated at a wildlife facility, and that a number of dead marine animals have been recovered.
The spill happened on the same stretch of beach as a 1969 spill - one that is credited with starting the American environmental movement.
In that incident, several hundred thousand gallons spewed from a blowout on an oil platform, resulting in the death of thousands of seabirds and marine mammals.
The alarm was first raised on Tuesday when authorities received reports of a foul smell near Refugio State Beach around midday (04:00 GMT).
Emergency responders found a half-mile slick in the ocean and traced the origin to a ruptured onshore pipeline spewing oil into a rainwater drain which ran to the ocean.
The pipeline was shut off about three hours later.