A pipeline rupture that spilled an estimated 101,000 gallons of crude oil near Santa Barbara last month occurred along a badly corroded section that had worn away to a fraction of an inch in thickness, federal regulators disclosed Wednesday.

The preliminary findings (PDF) released by the federal Pipeline and Hazardous Materials Safety Administration (PHMSA) point to a possible cause of the May 19 spill that blackened popular beaches and created a 9-mile slick in the Pacific Ocean. Gov. Jerry Brown declared a state of emergency in Santa Barbara County due to the effects of the spill.

The PHMSA findings were released as tar balls washed ashore and forced the closing of a four-mile stretch of beach about 100 miles south in Long Beach. There is no indication that the balls are related to the Santa Barbara spill or to the mysterious goo balls that closed Manhattan Beach for three days last week.

The agency said investigators found that there was "extensive external corrosion" at the break site. It had degraded the pipe wall thickness to 1/16 of an inch, and that there was a 6-inch opening near the bottom of the pipe. Additionally, the report noted that the area that failed was close to three repairs that had been made to the pipeline because of corrosion after 2012 inspections.

The agency documents said findings by metallurgists who examined the pipe wall thickness at the break site conflicted with the results of inspections conducted on that area of pipe on May 5 for operator Plains All American Pipeline. Those inspections pinpointed a 45 percent loss of wall thickness in the area of the pipe break, meaning they concluded the pipe was in far better condition.

Government inspectors "noted general external corrosion of the pipe body during field examination of the failed pipe segment," the report said.

Investigators found "this thinning of the pipe wall is greater than the 45 percent metal loss which was indicated" by the recent Plains All American inspections.

The agency ordered the company to conduct additional research and possible repairs on the line, which has been shut down indefinitely.

The findings indicate that over 80 percent of the metal pipe wall had worn away over time because of corrosion, said Richard Kuprewicz, president of Accufacts Inc., which investigates pipeline incidents.

"There is pipe that can survive 80 percent wall loss. When you're over 80 percent, there isn't room for error at that level," said Kuprewicz, who cautioned that it's still too soon to determine what caused the pipe to fail.

The company said in a statement that it is committed to working with federal investigators "to understand the differences between these preliminary findings, to determine why the corrosion developed and to determine the cause of the incident."

Plains All American said in a regulatory filing that there is no timeline to restart the line, which runs along the coast north of Santa Barbara. A company spokeswoman said there's no estimate yet of the cost of cleanup, which involves nearly 1,200 people.

The agency also ordered restrictions on a second stretch of pipeline, which the company had shut down May 19, restarted, then shut down again on Saturday.

That second line had similar insulation and welds to the line that spilled oil last month. It cannot be started until the company completes a series of steps, including testing.

California's U.S. senators issued a statement last week calling the response to the spill “insufficient” and demanding the pipeline company explain what it did, and when, after firefighters discovered the leak from the company's underground 24-inch pipe.

Plains All American and its subsidiaries operate 17,800 miles of crude oil and natural gas pipelines across the country, according to federal regulators. The company has been cited for 10 oil spills that violated the Clean Water Act and in 2010, Plains settled with the EPA after agreeing to pay $3.2 million in civil penalties.

Earlier this week, Stace Cheverez, a commercial fisherman, sued Plains, alleging the environmental disaster would cause decades of harm to the shore and hurt many businesses.

The suit alleging negligence and liability under state and federal laws does not specify damages but said they will exceed $5 million.

A Plains spokeswoman said the company does not comment on legal matters, but it is addressing claims filed against it. Company officials have apologized for the spill and defended its safety record.

Cheverez is seeking class-action status for fishermen and other businesses losing income because of the May 19 spill that dumped at least 101,000 gallons of oil, fouled beaches and killed at least 140 marine mammals and birds.

Al Jazeera and The Associated Press