Cutoff valves like the one that failed to stop the Gulf of Mexico oil disaster have repeatedly broken down at other wells in the years since federal regulators weakened testing requirements, according to an Associated Press investigation.

These steel monsters known as blowout preventers or BOPs — sometimes as big as a double-decker bus and weighing up to 640,000 pounds — guard the mouth of wells. They act as the last defense to choke off unintended releases, slamming a gushing pipe with up to 1 million pounds of force.

While the precise causes of the April 20 explosion and spill remain unknown, investigators are focusing on the blowout preventer on the Deepwater Horizon rig operated by BP PLC as one likely contributor.

To hear some industry officials talk, these devices are virtually foolproof.

But a detailed AP review shows that reliability questions have long shadowed blowout preventers:

_ Accident reports from the U.S. Minerals Management Service, a branch of the Interior Department, show that the devices have failed or otherwise played a role in at least 14 accidents, mostly since 2005.

_ Government and industry reports have raised questions about the reliability of blowout preventers for more than a decade. A 2003 report by Transocean, the owner of the destroyed rig, said: "Floating drilling rig downtime due to poor BOP reliability is a common and very costly issue confronting all offshore drilling contractors."

_ Lawsuits have fingered these valves as a factor in previous blowouts.

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