As underwater robots secured a massive dome over the main oil leak at the bottom of the Gulf of Mexico, investigators said that a methane gas bubble, followed by a series of safety failures, trigged the ecological disaster.

By Friday night, crews had lowered the 100-ton concrete and steel contraption onto the blown-out well.

Workers were then expected to take 12 hours to let the box settle and ensure its stability before robots could be used to attach a giant funnel. The funnel, it is hoped, will then guide the oil to the surface.

"It appears to be going exactly as we hoped," BP spokesman Bill Salvin told The Associated Press on Friday afternoon, shortly after the four-storey device hit the seafloor.

"Still lots of challenges ahead, but this is very good progress."

"We are essentially taking a four-story building and lowering it 5,000 feet and setting it on the head of a pin."

The robots are equipped with lights to cut through the darkness on the seafloor; the cameras have been kept out of the oil's path. Not much sea life is visible.

A crane began lowering the containment box on Thursday night, while undersea robots placed buoys around the main oil leak to act as markers to help line up the box.

The operation had to be delayed a few hours Thursday, because dangerous fumes were rising from the oily water on a windless night. The fear has been that a spark caused by scraping metal could ignite a fire.

A steel pipe will be installed between the top of the box and tanker. The device will then begin to collect as much as 85 per cent of the oil and funnel it up to a tanker waiting on the surface.

If all goes well, the whole structure could be operating by Sunday. But since nothing like this has ever been attempted before, it could take days before it's learned whether the plan has been successful.

"We haven't done this before," said oil company BP spokesman David Nicholas. "It's very complex and we can't guarantee it."

At depths of 1,500 metres, the water pressure is so intense, it's enough to crush a submarine. The fear is that if the dome is not positioned over the well accurately, or it could damage the leaking pipe and perhaps make the problem even worse.

Other risks include ice clogs in the pipes -- a problem that crews will try to prevent by continuously pumping in warm water and methanol. There is also the danger of explosion when separating the mix of oil, gas and water that is brought to the surface.

"I'm worried about every part, as you can imagine," said David Clarkson, BP vice president of engineering projects.

The blown-out Deepwater Horizon oil rig has been spewing an estimated 200,000 gallons a day since it exploded April 20. That oil has now reached the Chandeleurs barrier islands off the Louisiana coast, many of them with fragile animal habitats.

There are reports that several birds were spotted diving into the oily, rust-coloured water, and dead jellyfish washed up on the uninhabited islands.

The oil slicks stretch for kilometres off the Louisiana coast, where workers are trying to skim, corral and set the oil afire, while people watch from the shore.

With files from The Associated Press