U.S. federal regulators ordered the company whose pipeline spilled thousands of gallons of oil along a coastline in the western state of California to take corrective steps before it can restart the line.
The U.S. Transportation Department issued a corrective action order that requires Plains All American Pipeline to perform detailed and actions, including removal of the damaged section of pipeline, emptying the rest of the line and then testing it.
The company also must carry out in independent review of the inspection results and a "root-cause" analysis of the spill that explores the cause of the failure.
The spill was discovered Tuesday at Refugio State Beach in southern California near the city of Santa Barbara and is believed to have come from a rupture in an onshore, subterranean pipeline
Largest oil spills in the United States.
Officials estimate more than 380,000 liters (100,000 gallons) of oil may have leaked out before the oil flow was shut off late Tuesday, and about 79,800 liters (21,000 gallons) may have seeped down the beach and into the sea.
The 24-inch (61-centimeter) pipe, built in 1987, had no previous problems and was thoroughly inspected in 2012, according to Plains All American Pipeline. The pipe underwent similar tests about two weeks ago, though the results had not been analyzed yet.
The company has apologized and vowed to conduct cleanup operations around the clock. The cause of the break and subsequent spill, which has already killed some marine life, including lobsters, pelicans and kelp bass, may take weeks or even months to determine.
"Spill recoveries happen in stages," said Rick McMichael, a company official. "We are still in what we call the response and recovery stage."
Plains All American Pipeline, which has a number of safety and maintenance violations on its record, said crews have yet to excavate the broken piece of pipeline, which under the law must be done in the presence of federal regulators and a third party.
Federal officials will examine the pipe once it is excavated to find clues as to how it ruptured.
Meanwhile, some 400 people are working to remove contaminated water and soil around the ruptured pipe, and hundreds more are expected to join the effort in the coming days.
Experts say the size of the spill is equivalent to the volume of water the average American residence uses in a year. Coast Guard officials say winds and water currents make it a moving target, and that the slick, which spans about 15 kilometers of coastline, is one of the worst in California's history.
"Operations have continued non-stop since the onset of the oil spill and hundreds of responders are working diligently in every facet of the incident," said Jennifer Williams of the U.S. Coast Guard. "We are continuing to deploy the best assets available and are working with our emergency response partners at federal, state and local levels to improve conditions throughout the region."
On Wednesday, California Governor Jerry Brown declared a state of emergency and issued a proclamation that frees up state funding to assist with the cleanup. Thousands of liters of oil have been collected in cleanup efforts.
That area of coastline is known for one of the nation's biggest oil spills in 1969. That spill helped inspire the current U.S. environmental movement.
In addition to the environmental damage, California's tourist economy will suffer with beaches closed just before the popular Memorial Day weekend.
Some material in this report was provided by AP.