Experts were waiting to examine a key piece of evidence in the BP oil spill after the 300-ton blowout preventer was raised to the surface of the Gulf of Mexico early today.
Investigators may now be able to answer the most elusive question since the Deepwater Horizon rig explosion unleashed the massive oil spill more than four months ago - why didn't it stop the oil?
A crewman guided a crane to hoist the 50ft blowout preventer from a mile beneath the sea to the surface. It took more than 29 hours for the device to reach the surface of the Gulf.
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 ice-like crystals - called hydrates - formed on the blowout preventer. The device could not be hoisted safely from the water until the combustible hydrates melted, 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 crystals caused BP problems in May, when hydrates formed on a 100-ton, four-storey dome the company tried to place over the leak to contain it.
The April 20 explosion aboard the Deepwater Horizon killed 11 workers and led to 206 million gallons of oil spewing from BP 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 do not know exactly how or why the gas escaped or why the blowout preventer did not seal the well pipe at the sea bottom after the eruption as it was supposed to.
While the device did not close - or may have closed partially - investigative hearings have produced no clear picture of why it did not 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 politicians after the disaster that there was no evidence the device itself failed and suggested debris might have been forced into it by the surging gas.
There have also been claims that the blowout preventer did not undergo a rigorous recertification process in 2005 as required by US regulators.
Recertifying the device requires completely disassembling it out of the water and can take as long as three months to complete.
Evidence from BP and Transocean officials also showed that repairs were not always authorised by the manufacturer, Cameron International, and that confusion about the equipment delayed attempts to close the well in the days after the explosion.
Lawyers will be watching closely, too, as hundreds of lawsuits have been filed over the oil spill. Future liabilities faced by a number of corporations could be riding on what the analysis of the blowout preventer shows.
A temporary cap that stopped oil from gushing into the Gulf in mid-July was removed on Thursday. No more oil was expected to leak into the sea, but crews were standing by with collection vessels just in case.
The US government said a new blowout preventer had been placed on the blown-out well yesterday.
Officials wanted to replace the failed blowout preventer first to deal with any pressure caused when a relief well BP has been drilling intersects the blown-out well.
Once that intersection occurs some time after the Labour Day weekend, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.