This image taken from video provided by BP PLC on Saturday shows the blowout preventer from the Gulf oil well being raised to the surface. (BP PLC/Associated Press)

Investigators may be able to determine why a blowout preventer failed to stop the massive Gulf of Mexico oil spill more than four months ago as they get a close-up view of the key piece of equipment for the first time.

A crewman guided a crane Saturday to hoist the 15-metre, 305-tonne blowout preventer from 1.6 kilometres beneath the sea to the surface. It took about 29½ hours for the equipment to reach the surface of the Gulf at 7:54 p.m. ET.

FBI agents were among the 137 people aboard the Helix Q4000 vessel, waiting to escort the device 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, 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 PLC problems in May, when hydrates formed on a 100-tonne, 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 780 million litres 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.

Some experts have cautioned that the blowout preventer will not provide clues to what caused the gas bubble. And it's possible a thorough review may not be able to show why it didn't work.

That could leave investigators to speculate on causes using data, records and testimony before a congressional committee.

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 Thursday. No more oil was expected to leak into the sea, but crews were standing by with collection vessels just in case.

The government said a new blowout preventer was placed on the blown-out well late Friday. Officials wanted to replace the failed blowout preventer first to deal with any pressure that is caused when a relief well BP has been drilling intersects the blown-out well.

Once that intersection occurs sometime after Labour Day, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.