Print edition: 
ON THE GULF OF MEXICO — Investigators may now be able to answer the most elusive question since a rig explosion unleashed the massive Gulf of Mexico oil spill more than four months ago, as they get a close-up view of a key piece of equipment for the first time.
Why didn't it stop the oil?
A crewman guided a crane Saturday to hoist the 50-foot, 300-ton blowout preventer from a mile beneath the sea to the surface. It took about 29 1/2 hours for the blowout preventer to reach the surface of the Gulf at 6:54 p.m. CDT.
FBI agents were among the 137 people aboard the Helix Q4000 vessel, waiting to escort the device back to a NASA facility in Louisiana for analysis.
Crews had been delayed after icelike crystals — called hydrates — formed on the blowout preventer. The device couldn't be safely hoisted from the water until the hydrates melted because the hydrates are combustible, said Darin Hilton, the captain of the Helix Q4000.
Hydrates form when gases such as methane mix with water under high pressure and cold temperatures.
The April 20 explosion aboard the Deepwater Horizon killed 11 workers and led to 206 million gallons of oil spewing from BP PLC's undersea well.
Investigators know the explosion was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before igniting.
But they don't know exactly how or why the gas escaped. And they don't know why the blowout preventer didn't seal the well pipe at the sea bottom after the eruption, as it was supposed to. While the device didn't close — or may have closed partially — investigative hearings have produced no clear picture of why it didn't plug the well.
Documents emerged showing that a part of the device had a hydraulic leak, which would have reduced its effectiveness, and that a passive "deadman" trigger had a low, perhaps even dead, battery.
Steve Newman, president of rig owner Transocean, told lawmakers following the disaster that there was no evidence the device itself failed and suggested debris might have been forced into it by the surging gas.
