Californian politicians are demanding answers over why it took several hours before a pipeline operator reported a major oil spill to federal authorities - and the subsequent lag before the public was notified of the danger.
Investigators are still searching for what caused the offshore pipeline to rupture, sending the equivalent of more than 3000 barrels of crude oil into the Pacific Ocean.
The spill soiled the coastline and forced officials to close beaches in several cities in Orange County, just south of Los Angeles.
The US Coast Guard and drilling company Amplify Energy came under further scrutiny about the time it took to respond to the spill, amid reports mariners in the area first saw oil in the water on Friday night local time.
Official notification to the National Response Center did not come until Saturday around midday.
Amplify's Chief Executive Martyn Willsher insisted the company was not informed of a spill until Saturday morning.
"If we were aware of something on Friday night I promise you we would have immediately stopped all operations," he said.
A letter from the US Pipeline and Hazardous Materials Safety Administration to Amplify on Monday noted the line ruptured at about 5.30am local time on Saturday, but Amplify subsidiary Beta Offshore did not shut the line for roughly three-and-a-half hours.
Willsher said the company is working with investigators to determine whether there was a loss of pipeline pressure that should have triggered leak detection warnings.
"The pipeline reportedly should have been monitored by an automated leak detection system," wrote Democratic members of Congress in a letter on Wednesday.
Tom Umberg, a state Senator who represents the coastal region, told a news conference on Tuesday officials needed to explain why the response was delayed, and whether the pipeline had been adequately inspected.
"It's very difficult for us to understand how this could occur," he said.
The pipeline that burst is supposed to be inspected every two years, according to the US Bureau of Safety and Environmental Enforcement, which monitors offshore activity.
The pipeline's last internal inspection was October 2019, BSEE said.
That inspection discovered eight anomalies, of which the two largest were reported as repaired, according to a memorandum detailing a 2019 inspection of the Beta Offshore pipeline.
The oil appears to have leaked through a 33cm gash in the pipe, which was situated about 32m from where it should have been, Willsher said on Tuesday.
A 1.2km section of the 28.5km pipeline was displaced from its official location according to mapping data, officials said.
The coast guard is investigating whether a vessel in the vicinity a few days earlier could be responsible, according to the Los Angeles Times.