US Congressional investigators believe the device meant to stop oil leaking from a Gulf of Mexico well after last month’s rig explosion was faulty.
The blowout preventer (BOP), a set of huge valves, had a hydraulic leak and a failed battery, they said after studying BP and other documents.
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.
Introducing the American Power Act, Senator John Kerry described it as "a bill for energy independence after a devastating oil spill" and "a bill to hold polluters accountable".
President Barack Obama hailed the legislation, which faces a difficult passage.
"The challenges we face – underscored by the immense tragedy in the Gulf of Mexico – are reason to redouble our efforts to reform our nation’s energy policies," Mr Obama said.
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.
‘Apparently defective’
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.
"The safety of [Deepwater Horizon’s] entire operations rested on the performance of a leaking and apparently defective blowout preventer," he told a House hearing.
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?" Representative Stupak asked.
The hearing was also told that the BOP had been modified, which made it difficult to operate after the accident.
Its emergency back-up controls may have failed because the explosion that destroyed the rig also disabled communications preventing workers from sending signals underwater, the hearing heard.
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, told the hearing it was "far too early" to draw conclusions on what had caused the accident. He added that the company had not been able to the examine the BOP.
Ambitious bill
The headline 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 – 12 May
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