Deepwater Horizon blowout preventer 'faulty' - Congress
The blowout preventer was built by Cameron International
The device meant to stop oil leaking from a Gulf of Mexico well after last month's rig explosion was faulty, US Congressional investigators have said.
The blowout preventer (BOP), a set of huge valves, had a hydraulic leak and a failed battery, they said after studying documents from BP and others.
Oil industry chiefs say it is too early to conclude what caused the disaster.
A climate change bill just submitted to the Senate could allow states to veto some offshore drilling plans.
Meanwhile, President Barack Obama has also proposed new legislation which foresees $188m (£127m) in emergency funding to cope with the fallout from the oil spill.
His administration intends to recover most of the money from BP.
In a letter to House speaker Nancy Pelosi, Mr Obama requested $100m in immediate funding for the Coast Guard and $29m for the Secretary of the Interior for additional inspections, enforcement and studies, saying he would "spare no effort to clean up whatever damage has been caused".
Eleven people died when an explosion - thought to have occurred after a surge of methane gas from deep within the well - destroyed the Deepwater Horizon rig on 20 April.
At least 4m gallons (15m litres) of oil have leaked into the Gulf from the damaged well to date, the Associated Press news agency reports, and desperate efforts are being made to protect the Gulf coast's ecosystem from the slick.
Attempts to stop the leak have so far failed but engineers hope at least to begin reducing it later this week with a steel dome - dubbed the "top hat" - which has been lowered on to the seabed.
Representative Bart Stupak, head of the House Subcommittee on Oversight and Investigations, said its investigators had uncovered significant problems with the BOP, which was meant to stop sudden, dangerous rushes of volatile oil and gas.
He said a 2001 report by Transocean, which operated the drilling rig on behalf of BP, indicated there could be as many as 260 failure possibilities in the equipment.
"How can a device that has 260 failure modes be considered fail-safe?" Mr Stupak asked.
The hearing was also told that the BOP had been modified, which made it difficult to operate after the accident.
A senior BP executive, Lamar McKay, told the House: "It's inappropriate to draw any conclusions before all the facts are known."
Jack Moore, president of Cameron International Corp, which built the BOP, also told the hearing it was "far too early" to draw conclusions, adding that the company had not been able to the examine the BOP.
Introducing the new climate change bill, Senator John Kerry described it as "a bill to hold polluters accountable" after the "devastating" oil spill.
The main aim of the American Power Act is to cut greenhouse gas emissions by 17% within a decade and by 80% by 2050, which is ambitious by any standards, the BBC's Madeleine Morris reports from Washington.
To try to achieve this, the bill pushes nuclear power and clean coal technology by giving financial incentives.
It imposes a strict carbon-trading market as well as mandatory cuts on emissions by certain manufacturers and power companies.
Because of the Gulf of Mexico spill, a provision was added at the last minute, allowing states to veto drilling less than 75 miles (120km) off their own coastlines.
But that may not placate several Democratic senators, who are already uncomfortable with offshore drilling and, because of the Deepwater Horizon disaster, are now even more so, our correspondent says.
Republican support for the act is by no means guaranteed at the moment, she adds.
How the oil has spread Approximate oil locations 22 April - 15 May
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