Kenner, Louisiana (CNN) -- A "black rain" of drilling fluid and a roar of escaping gas erupted from the doomed Deepwater Horizon shortly before the explosion that sank the oil rig, the captain of a nearby ship testified Tuesday.
Alwin Landry's supply vessel Damon Bankston was alongside Deepwater Horizon at the time of the blast. Landry said the first sign of trouble was when drilling "mud" -- a mixture used to pressurize and lubricate the drills -- began falling onto the stern of his ship.
"We essentially closed the wheelhouse doors. I went to the port side, and I looked out up at the derrick. That's when I see the mud coming out of the top of the derrick," Landry told investigators Tuesday.
When he radioed the rig's bridge, Landry said, its captain told him there were problems with the well and he should move his ship away. Seconds later, he said, he heard "the percussion and the slight flash of green" of an explosion.
Deepwater Horizon sank April 22, two days after the explosion. The blast left 11 people aboard the rig presumed dead and uncorked a gusher of oil that has been spewing an estimated 210,000 gallons (5,000 barrels) of crude oil a day into the Gulf of Mexico.
Landry's testimony came on the first of two days of hearings by a joint Coast Guard-Interior Department panel.
His ship picked up the rig's survivors, and Landry said the rig's captain, Curt Kutcha, told him he had tried to activate a "kill switch" that would cut off the well before abandoning the structure.
The crew did not know whether it was working when they fled the burning rig, Landry told investigators.
The federal Minerals Management Service, the Interior Department agency that oversees offshore oil exploration, inspects drill rigs once a month, MMS inspector Eric Neal told the panel. Test results from the rig's blowout preventer, a massive device meant to cut off the well in an emergency, and drilling mud weights are reviewed as part of that process, he said.
Neal inspected the rig April 1 and found no violations, he testified. But he did not inspect the preventer's emergency disconnect system, he said.
The companies involved in the drilling process are blaming each other for the disaster.
The chairman of well owner BP's American subsidiary, Lamar McKay, told a U.S. Senate committee Tuesday that rig owner/operator Transocean was responsible for the rig's operation and for testing its blowout preventer.
But Transocean CEO Steven Newman told the Senate Energy and Natural Resources Committee that the well's cementing, or the casing that holds the well in place, must have failed. BP, as the well's owner, was responsible for designing the casing, while oilfield services contractor Halliburton performed the cementing.
"Without a failure of one of those elements, the explosion could not have occurred," Newman said.
And Halliburton's chief safety and environmental officer, Tim Probert, told the committee that responsibility lay with either Transocean or BP.
During the cementing of the well, Halliburton simply followed BP's instructions, he said, and the blowout preventer was Transocean's responsibility.