– Flaws in safety equipment and procedures used on Gulf of Mexico rigs persist four years after 11 workers died in an explosion aboard the Deepwater Horizon that led to the largest offshore oil spill in U.S. history.

Drillers and regulators misunderstand the limits of blowout preventers, or BOPs, which are supposed to pinch pipes and prevent oil and natural gas from escaping a well in an emergency, the U.S. Chemical Safety and Hazard Investigation Board said in a report made public Thursday. The blowout preventers on BP PLC’s Macondo well failed, causing millions of gallons of oil to spew into the Gulf in April 2010.

Industry practice and federal safety rules currently in place in the Gulf may not prevent another catastrophic spill, according to the report. U.S. regulations fall short of standards used for drilling off the coasts of Norway, Australia and Britain, which require more rigorous, regular and independent safety-equipment checks, the agency said.

“Vital aspects of effective safety critical element management have not been established in the U.S. offshore,” the report concluded. “Deficiencies identified during the failure analysis of the Deepwater Horizon BOP could still remain undetected in BOPs currently being deployed to well heads.”

Each system in the five-story-tall BOP must be tested individually so that a properly operating backup system doesn’t mask the failure of another, according to the report. A predeployment test of the Deepwater Horizon BOP failed to reveal that two backup systems were wired improperly, investigators said.

The government halted deep-water exploration drilling in the Gulf for five months after the Macondo disaster. The agency that oversaw offshore drilling was split to improve safety and enforcement, and regulations were strengthened to prevent a recurrence.

“There’s nothing here that hasn’t already been exhaustively addressed by regulators and the industry,” said Brian Straessle, a spokesman for the American Petroleum Institute, an industry group. “The report ignores the tremendous strides made to enhance the safety of offshore operations.”