LOS ANGELES (CNN) — A pipeline that ruptured, spilling more than 100,000 gallons of heavy crude oil last month on a California state beach and into the ocean, was badly corroded, a preliminary government report said Wednesday.

The Refugio State Beach in Santa Barbara is a scenic recreational park, and the spill has shut it down until at least June 18. Crews are still working to clean it up, according to the state’s Parks and Recreation Department.

Though the cause of the spill at and near the park is still under investigation, preliminary findings revealed that Plains All American Pipeline, which runs the pipe, had discovered two weeks before the spill that the stretch had four troublesome spots.

Three of these showed “extensive external corrosion, primarily on the bottom quadrant of the pipe,” according to the preliminary report by pipeline specialists at the U.S. Department of Transportation.

In the worst spots, corrosion had eaten away 54% to 74% of the pipe wall, according to Plains’ May 5 assessment. The company said that the area of the pipe that failed had lost 45% of its wall thickness.

Government inspectors also found the pipeline stretch to be generally corroded. The rupture that caused the spill was about 6 inches long and at the bottom of the pipe, where corrosion had eaten away all but a sixth of an inch of the metal.

Plains spokesman Patrick Hodgins had mentioned his company’s assessment after the May 19 spill, saying then that its results were still under review.

Plains All American is among the worst violators listed by the U.S. Pipeline and Hazardous Materials Administration, which is part of the Transportation Department and issued the preliminary report.

Plains surpassed all but four of more than 1,700 operators in safety and maintenance infractions, the federal agency said.

Hodgins suggested the comparison wasn’t fair because “we’re also much larger than those companies that we were compared to.” The others are about half his company’s size, he said.

Plains has had 175 federal safety and maintenance violations since 2006, responsible for more than 16,000 barrels of spills that have caused more than $23 million worth of property damage.

Plains All American Pipeline has been committing money to safety improvements for the past seven years, said Pat Hutchins, the company’s senior director of safety.

Plains violated federal environmental regulations 10 times between 2004 and 2007, when about 273,420 gallons of crude oil were discharged into waters or shorelines in Texas, Louisiana, Oklahoma and Kansas, the Environmental Protection Agency said.

Most of the spills were caused by pipe corrosion, the EPA said.

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