Fire boats battle the blazing remnants of the Deepwater Horizon oil rig on April 21, 2010. A stuck pipe was to blame for impeding the blowout preventer system, according to a recently released report. (HO/AFP/GETTY IMAGES)

It was the mystery that wouldn’t go away after the oil stopped gushing: Why did the blowout preventer of the drilling rig Deepwater Horizon fail to prevent the blowout of April 20, which killed 11 people and led to the historic BP oil spill?

The official answer came Wednesday, and it was both strikingly mundane and, at least for critics of the petroleum industry, alarming.

The forensic analysis of the blowout preventer, or BOP, conducted under contract to federal investigators by the Norwegian company Det Norske Veritas showed that the initial loss of well control — and the violent surge of oil and gas up the well — caused the drillpipe to buckle and move slightly off center. That fouled the operation of the blind shear rams, the blades designed to close on the drillpipe and shut in the well.

This final line of defense was overmatched by the blowout once it began. When the blades closed on the pipe, the misalignment prevented the shearing surfaces of the rams from achieving a clean cut. Moreover, the rubber “packers” around the blades were unable to make a tight seal. The drillpipe snagged the rams in a partially open position.

Oil and gas then continued to surge to the surface, fueling the fire that sank the rig and led to the worst oil spill in U.S. history.

The BOP’s failure was just one element in a complex sequence of decisions and mistakes by BP and its contractors that led to the disaster, according to multiple investigations. But the new report calls into question not just the specific companies and actions involved, but also the ability of all of these pieces of hardware to function as advertised.

“Blowout preventer is a misnomer,” said an engineer who assisted in the probe and who asked for anonymity because he was not authorized to speak for the investigation. “People have been thinking of this as a fail-safe device, and it’s more of an operating device.”

This finding is likely to incite calls for caution in granting deepwater drilling permits just as the Obama administration has started to issue some again. Regulators recently granted a deepwater drilling permit for a well that will be operated by Noble (with BP as a major equity partner) and to a drilling plan for Shell. Chevron has filed permit applications with the first decision due by April 8.

Rep. Edward J. Markey (D-Mass.), a sharp critic of BP since the spill, called for a review of every BOP currently operating in U.S. waters. “A blowout preventer is like a car’s airbag. It can’t prevent the car accident, but it is supposed to deploy and prevent fatalities,” Markey said. “This report calls into question whether oil industry claims about the effectiveness of blowout preventers are just a bunch of hot air.”

Transocean, owner of the rig and the BOP, contended that the report provided partial vindication. The company, which has more than 100 deepwater rigs around the world, noted that the report found no fault in the maintenance of the BOP, which had been a subject of great acrimony by BP and Transocean lawyers.

“The findings confirm that the BOP was in proper operating condition and functioned as designed,” said Transocean. It said well conditions “exceeded the scope of BOP’s design parameters.”

BP reacted more cautiously.

“We support efforts by regulators and the industry to make BOPs more reliable and effective,” the company said in a statement.

A spokesman for Cameron, maker of the rig’s BOP, said it “was designed and tested to industry standards and customer specifications.”

The probe of the 380-ton BOP by the Interior Department’s Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE) was not free of controversy. BOEMRE hired DNV to do the post-mortem even though Transocean hired DNV in 2007 to inspect safety procedures on the Deepwater Horizon.

The handling of the investigation drew sharp criticism from the Chemical Safety Board, another federal agency and one with experience investigating industrial accidents. Interior said it acted appropriately.

The offshore industry has long believed that it has redundancy in its safety measures, with the BOP itself a prime example. There are multiple ways that the blind shear rams can be activated — including an automatic mode known as the “dead-man” switch.

But the forensic analysis has found a way that all the redundancies can be overcome simultaneously — a single point of failure. If the pipe is misaligned and the shearing blades can’t close neatly on the pipe, it does not matter how many techniques are available for activating the rams, the report says.