British energy giant BP on Wednesday lowered the "top hat" oil containment box into the Gulf of Mexico in a second attempt to cap the oil gushing from a ruptured undersea wellhead.

The device has reached the sea floor and should be in position over the wellhead and operational by the end of the week, BP said.

A 100-ton, four-story containment vessel designed to cap the larger of two leaks in the well was rendered useless on Sunday when ice-like hydrate crystals formed in its domed roof and blocked the top, making it buoyant and clogging its nozzle.

The "top hat," a 5-foot-tall, 4-foot-diameter structure, weighs less than 2 tons, would keep most of the freezing cold seawater out at the beginning of the capping process and would allow engineers to pump in methanol to keep the hydrates from forming, said BP's chief operating officer Doug Suttles.

U.S. Interior Secretary Ken Salazar said the success of the second attempt to cap the well would be known within a few days, according to CNN.

"On this Thursday, we should know whether or not this alternative top hat cofferdam is going to work," Salazar told CNN. "And then the next key date is Saturday, because by Saturday they will have the diagnostics completed through X-rays and gamma rays and pressure ratings to be able to make decisions about what the next steps are ... ."

If the second attempt fails, BP said it may try an alternative operation to inject golf balls, tires and other "junk" under high pressure into the main leak, 1,500 meters down on the seabed, and then jam it up.

The "Deepwater Horizon" drilling rig, owned by Swiss-based Transocean and leased by BP, sank April 22 some 52 km off Venice, Louisiana, after burning for roughly 36 hours. The untapped wellhead continues gushing oil into the Gulf of Mexico.