April 20, later catching fire and sinking in a spectacular incident that claimed the lives of 11 workers and has spilled millions of gallons of crude into the sea.
On the second day of congressional hearings into the accident on Wednesday, US lawmakers pressed big oil about flaws in a key safety device that was fitted on the rig and meant to stop sudden, dangerous rushes of volatile oil and gas.
Frustrated by three weeks of failed efforts to stop the massive slick, President Barack Obama sent top experts to aid BP in its battle to cap the offshore oil platform's ruptured well.
Energy Secretary Steven Chu, a Nobel prize-winning physicist, told reporters the "intellectual horsepower of the country is engaged in solving this problem," as an estimated 210,000 gallons of oil gushing daily from the well threatens the region's fragile ecosystem.
"Things are looking up," Chu said after he and Interior Secretary Ken Salazar met with BP engineers and executives at the British oil giant's command center in Houston, Texas. "Progress is being made."
Without going into detail, Chu cautioned the situation was still not under control, but said: "I'm feeling more comfortable than I was a week ago."
The latest attempt by engineers to plug the leak from the rig, lying 50 miles (80 kilometers) off Louisiana's coast, features a streamlined 1.6-tonne structure dubbed a "top-hat" now lying on the seabed.
Officials hope the smaller structure, equipped to allow warm water and methanol to circulate inside, will not develop the same icy crystals that scuttled Sunday's attempt to cap the spill with a bigger dome.
BP began drilling a relief well on May 2 that could divert the flow until the well is permanently sealed, but this may not be ready until August.
While official investigations continue into the disaster, House Energy and Commerce Committee chairman Henry Waxman zeroed in on a valve intended to serve as the rig's safety mechanism.
"We have learned from Cameron, the manufacturer of the blowout preventer, that the device had a leak in a crucial hydraulic system and a defectively configured ram," designed to seal off an oil well's pipes, Waxman said.
Democratic Representative Bart Stupak said the leak was found in the hydraulic systems that provides emergency power to the shear rams intended to cut the drill pipe and seal the well.
Waxman also slammed Halliburton, condemning the oil services supplier's assertion that critical cementing work it had carried out on the platform had passed important safety tests as an "incomplete account."
"The well did pass positive pressure tests, but there is evidence that it may not have passed crucial negative pressure tests," just hours before the explosion, he said.
BP America president Lamar McKay cited "anomalous pressure test readings," noting "that discrepancy is critical, and the investigation will have to tear that apart piece by piece, absolutely."
Stricken communities along the southern Gulf coast have already seen their livelihoods damaged by a ban on fishing. In a bid to mitigate the impact, Louisiana officials reopened a small stretch of the fishing grounds that did not appear under immediate threat.
But clean-up teams said oil had been found washing up on Whiskey Island, just 18 miles (25 kilometers) off the coast, after hitting the protected Chandeleur Islands last week east of New Orleans.
Tar balls, already found on Alabama beaches, were also washing up in Louisiana, as Florida issued emergency orders in case the giant oil slick strikes its popular beaches.
Some 1.5 million feet of boom have been laid so far in the gulf to try to contain the slick, amid forecasts that stronger winds could push the slick closer to the shore.
But some of the boom washed up uselessly on shore.
The floating barriers "seems mostly to put on a show," fishing boat captain Carey O'Neil said.
"It only works in ideal conditions. Even if it's anchored right, which I don't see it being, it just needs a swell or a few waves and it gets pushed up or onto the shore. With some waves the oil gets pushed right over them."