"Significant pressure discrepancies were observed in at least two of these tests, which were conducted just hours before the explosion," Waxman said.
Asked about the tests, Steven Newman, president of Transocean, which owned the drilling rig, and Lamar McKay, president of BP America, told the committee the pressure readings were worrisome.
They indicated "that there was something happening in the well bore that shouldn't be happening," said Newman. McKay said the issue was "critical in the investigation" into the cause of the accident.
The April 20 explosion 64km off the Louisiana coast killed 11 people and the sinking of the rig two days later ruptured the riser, unleashing a massive oil spill that after three weeks remains uncontained and threatens sensitive ecological marshes and wetlands as well as the region's fishing industry.
More than 5,000 barrels of oil a day are believed to be pouring into the Gulf of Mexico, and experts say the spill could become one of the worst environmental disasters in decades.
McKay cautioned that "it's inappropriate to draw any conclusions before all the facts are known" but Waxman said important elements of what went wrong were beginning to surface.
The congressman said it appeared clear that there were problems with the blowout preventer before the accident and confusion almost right up to the time of the explosion over the success of a process in which cement is injected into the well to temporarily close it in anticipation of future production.
|Anger is growing three weeks into the uncontained oil spill [AFP]
At the House Energy and Commerce Committee hearing into the spill, Bart Stupak, another Democratic congressman, said there were at least "four significant problems with the blowout preventer".
Stupak said when an underwater remote vehicle tried to activate the blowout protector's devices designed to ram through the pipe and seal it, a loss of hydraulic pressure was discovered in the device's emergency power component.
When dye was injected "it showed a large leak coming from a loose fitting", said Stupak, citing BP documents.
Stupak also questioned why the safety equipment had been modified.
Newman, the Transocean executive, told the committee that the blowout preventer had been modified in 2005 at the request of BP and with approval of the government's Minerals Management Service.
Stupak said the committee had been told that one of the preventer's ram drivers had been changed so it could be used for routine testing and was no longer designed to activate in an emergency.
He said after the spill BP "spent a day trying to use this ... useless test ram".
|Drilling of a second well to divert flow from the damaged one is set to take months [Reuters]
Meanwhile on Wednesday, BP lowered a second containment box on to the sea floor near the well leak.
The dome could begin trapping oil from the leaking oil well next week, a spokesman for BP said on Thursday.
London-based BP has changed its timing for placing the undersea dome, after a company executive on Wednesday told reporters the device could be in place by late Thursday.
"Probably toward the end of next week is when we will start to move it and put it over the leak," Jon Pack, a BP spokesman, said. "So nothing particularly noteworthy is happening with that right now."
The so-called "top hat", designed to funnel oil from the leak about 1,500m deep to a vessel on the surface, is BP's second attempt to cap the well.
Another containment box was lowered onto the Deepwater Horizon oil rig last week, but small ice-like crystals formed insideof it and made the box too buoyant to stay in place.
The new box is smaller, so less cold water can accumulate inside and freeze. It also has a system that allows engineers to pump in methanol, a chemical which can be used as an anti-freezing agent.
If the second box fails, BP officials say they will try shooting debris such as golf balls and shredded tyres into the rig's pipes, in the hope of clogging the well.
BP is also drilling a second well to divert flow from the damaged one, but the company says it will take months to drill down 5.5km into the oil reserves; a spokesman said that the relief well has currently been drilled to about 3,000 metres.
In other developments on Wednesday, the White House asked congress to raise the limits on BP's liability to cover damage from the spill beyond the current $75m legal cap.
It also wants oil companies to pay more into a federal oil spill cleanup fund.
BP's McKay has said the company will pay any legitimate claim of damages beyond cleanup costs, despite the current federal cap.