ON THE GULF OF MEXICO (AP) — BP crews faced delays Saturday as they slowly raised the 300-ton blowout preventer that failed to stop oil from spewing into the Gulf of Mexico, taking care not to damage or drop a key piece of evidence in the spill investigation.

The device was about 500 feet from the surface and likely wouldn't be hoisted onto the deck of the Helix Q4000 vessel until about 7 p.m. EDT, said BP PLC spokesman Neil Chapman. Darin Hilton, the vessel's captain, said crews have to wait for icy hydrates to melt off the contraption before it can be brought aboard.

FBI agents are on board waiting to take possession of the device — a key piece of evidence in the investigation into the Gulf oil spill — after its mile-long journey. It will eventually be taken to a NASA facility in Michoud, La., to be analyzed.

The 50-foot device was detached from the wellhead Friday afternoon. Another blowout preventer had successfully been placed on the blown-out well, the government said later.

The April 20 explosion aboard the Deepwater Horizon killed 11 workers and led to 206 million gallons of oil spewing from BP'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 — hearings have produced no clear picture of why it didn't plug the well.

Lawyers will be watching closely, 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.

The raising of the blowout preventer followed Thursday's removal of a temporary cap that stopped oil from gushing into the Gulf in mid-July. No more oil was expected to leak into the sea, but crews were standing by with collection vessels just in case.

The government 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 Labor Day, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.