WASHINGTON -- As crews lowered a second dome into the Gulf of Mexico on Wednesday in BP's latest bid to stop the underwater oil leak, lawmakers on Capitol Hill and a federal investigative panel in Louisiana continued a second day of hearings that shed more light on the events -- and potential oversights -- leading to the explosion and sinking of the Deepwater Horizon offshore drilling rig.

U.S. Rep. Henry Waxman, D-Calif., said his committee's investigation into the Gulf oil spill revealed that a key safety device, the blowout preventer, had a leak in a crucial hydraulic system, The Associated Press reported.

Waxman said in a hearing Wednesday that the investigation also discovered that the well had failed a negative pressure test just hours before the April 20 explosion.

Waxman cited BP documents received by the Energy and Commerce Committee that showed there was a breach in the well integrity that allowed methane gas and possibly other hydrocarbons to enter the well.

The explosion and fire left 11 dead and mangled the deepwater well that continues to spew 210,000 gallons of crude oil into the Gulf of Mexico daily.

As the inquiries continued, the White House asked Congress to take up a $118 million oil spill response package, which includes a proposal to lift the current $75 million cap on liability for responsible parties, in this case BP.

Senior Obama administration officials said they would negotiate with Congress over the new liability limit, said Jeff Liebman, acting deputy director of the Office of Management and Budget.

The White House emphasized that it intends to hold BP to a pledge made during Tuesday's Senate hearing by Lamar McKay, the president of the company's U.S. operations.

Said McKay: "Let me be really clear: Liability, blame, fault, put it over here. We are dealing with ... we are the ... we are a responsible party."

On Wednesday, BP officials said they had placed a new "top hat" containment dome on the sea floor near the main leak, about 50 miles off Louisiana's coast.

The concrete and steel dome, 4 feet in diameter and 5 feet tall, will be lowered over the main leaking pipe, and the oil captured inside will be pumped to a barge on the surface.

The "top hat" strategy is similar to BP's failed effort to place a 78-ton steel and concrete cofferdam the size of a four-story house over the pipe and into the sea floor.