A crane hoisted a key piece of oil spill evidence to the surface of the Gulf of Mexico, giving investigators their first chance to personally scrutinise the blowout preventer, the massive piece of equipment that failed to stop the gusher four months ago.

It took 29 hours to lift the 15.24 metres, 300-ton blowout preventer from a mile beneath the sea to the surface. The five-story high device breached the water’s surface at 2354 GMT, and looked largely intact with black stains on the yellow metal.

FBI agents were among the 137 people aboard the Helix Q4000 vessel, taking photos and video of the device. They will escort it back to a NASA facility in Louisiana for analysis.

The blowout preventer was placed into a metal contraption specifically designed to hold the massive device. As it was maneuvered into place, crew members were silent and water dripped off the device.

Crews had been delayed raising the device after ice like crystals – called hydrates – formed on it. The device couldn’t be safely lifted 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 crystals caused BP PLC problems in May, when hydrates formed on a 100-ton, four-story dome the company tried to place over the leak to contain it.

As a large hatch opened up on the Helix to allow the blowout preventer to pass through, several hundred metres of light sheen could be seen near the boat, though crews weren’t exactly sure what it was.

The 20th April  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.