WASHINGTON (AP) — 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.

Investigators have long known that the device failed. But "The problems with this blowout preventer were worse than we understood," safety board managing director Daniel Horowitz said in an interview. "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.

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.

Robert Bea, a professor of engineering and expert in oil pipelines at the University of California Berkeley, praised the report and said blowout preventers are like cruise ship lifeboats, used only in last resort but crucial. In this case the blowout preventers "are deeply flawed, they've got holes," said Bea, who was not involved in the new study.

Kevin Ewing, a Washington attorney who represents many oil drilling interests, said well operators have improved their techniques. He cautioned against broad indictments on the safety of currently operating blowout preventers.

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 new 166-page 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.

Mulcahy said individual tests were suggested by the preventer's manufacturer but the companies instead followed a standard set by the industry.

The safety board also found that the drill pipe bent far earlier in the accident and from a different cause than determined by a presidential oil spill commission. That failure is also connected to a problem with the blowout preventers, investigators said.

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.

Donald Boesch, a University of Maryland professor who was on the presidential oil spill commission, agreed with the latest investigation. He said the chemical safety board was able to do what his board didn't do, a hands-on testing of the device.

The two companies involved in rig operations blamed each other. BP spokesman Geoff Morrell said all of the evidence "demonstrates that Transocean owned the rig's blowout preventer and was responsible for its maintenance."

Transocean spokesman Brian Kennedy noted that BP pleaded guilty to 12 felony counts from the accident while Transocean did to only one misdemeanor violation of the Clean Water Act. He also said the blowout preventer "had been tested successfully in accordance with regulatory requirements and activated as intended at the time of the incident, but was unable to seal the well because immense pressure buckled the drill pipe."