Engineers in the US have been assessing the piece of equipment that could show the cause of the worst oil spill disaster in the country's history.

On the weekend, they hoisted the so-called blow-out preventer out of the Gulf of Mexico and have been studying it for flaws.

The 300-ton device, which was supposed to seal off BP's Deepwater Horizon oil well, but failed during the emergency, was brought up by a giant crane on Saturday.

Because the device failed to work, 11 workers were killed and massive amounts of oil were released when the rig exploded on the 20th April.

Federal investigators have also begun looking over the equipment, taking photographic and video evidence to present to the US Justice Department's investigation into the oil spill.

On Saturday, the US official overseeing the response to the oil disaster, Coast Guard Admiral Thad Allen, said the BP well is no longer a threat after BP engineers finished installing a new safety valve on the partially-sealed well.

BP crews are expected to complete drilling a relief well later this week to help permanently "kill" the well.