Oil is seen pouring out of several spots near BP's blowout preventer on June 3, 2010. (BP Plc/Associated Press)

The broken blowout preventer whose failure allowed millions of litres of crude oil to spew freely into the Gulf of Mexico was removed Friday from a BP well on the ocean floor.

Underwater robots detached the 15-metre, 305-tonne device from the wellhead at 1:20 p.m CDT, a BP spokesman said Friday.

The device was being lifted slowly to the surface and wouldn't likely reach the top until sometime Saturday.

Earlier in the day, a vessel latched onto the equipment 1,500 metres below the water's surface.

Undersea video showed the device suspended in the water. A crane on the Helix Q4000 was being used for the task.

Key piece of evidence

The blowout preventer is considered a key piece of evidence in the investigation into what caused the explosion aboard the Deepwater Horizon drilling rig on April 20 that killed 11 workers.

After the rig sank on April 22, more than 750 million litres of oil spewed from the undersea well to which it had been attached.

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 in mid-July stopped the oil gushing into the Gulf. No more oil was expected to leak into the sea, but crews were standing by with collection vessels just in case.

The government wants 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 Sept. 6, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.