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WASHINGTON — A federal board investigation into the 2010 BP oil spill concludes that a last-ditch safety device on the underwater well had multiple failures, wasn't tested properly and still poses a risk for many rigs drilling today.

The report issued Thursday by the U.S. Chemical Safety Board zeroes in on what went wrong with the blowout preventer and blames bad management and operations. They found faulty wiring in two places, a dead battery and a bent pipe in the hulking device. And that, they said, led to the dumping of 172 million gallons of oil into the Gulf of Mexico and the nation's worst offshore oil disaster.

"The problems with this blowout preventer were worse than we understood," safety board managing director Daniel Horowitz said. "And there are still hazards out there that need to be improved if we are to prevent this from happening again."

The massive spill followed an explosion that killed 11 workers at the Deepwater Horizon drilling rig, about 50 miles off the Louisiana coast.

The blowout preventer that failed to stop oil from spewing into the Gulf of Mexico being raised to the surface. The last-ditch safety device that didn't stop the 2010 BP oil spill had multiple failures, wasn't tested properly, and still poses a risk for many rigs drilling today, another federal investigation board concludes.BP PLC / AP, file

Massive blowout preventers are anchored to the top of underwater wells. In an emergency, the devices use multiple mechanisms — including clamps and shears — to try to choke off the oil flowing up from a pipe and disconnect the rig from the well. They can operate automatically when pressure or electricity is cut off or manually.

The 9-year-old one that failed was nearly 57 feet tall and weighed about 400 tons.

Various investigations have found that the cause of the initial explosion involved multiple screw-ups with cement, drilling mud, fluid pressure, botched tests, management problems and poor decisions. The blowout preventer sealed the well temporarily, but then it failed and that caused the massive spill, the report found.

The report faulted the rig operators. The problem, said safety board investigator Mary Beth Mulcahy, was that well owner BP and rig operator Transocean didn't test the blowout preventer's individual safety systems. They just tested the device as a whole. It turned out there were two sets of faulty wiring that caused problems and a dead battery.

The board said the same device design is being used on at least 30 rigs worldwide and some general problems with operations and testing could affect other types of preventers.

— The Associated Press