The deadly blowout of an oil rig in the Gulf of Mexico was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding, according to interviews with rig workers conducted during BP's internal investigation.

While the cause of the explosion is still under investigation, the sequence of events described in the interviews provides the most detailed account of the 20 April blast that killed 11 workers and left more than three million gallons of crude pouring into the Gulf.

Portions of the interviews were described in detail by Robert Bea, a University of California Berkeley engineering professor who serves on a National Academy of Engineering panel on oil pipeline safety and worked for BP PLC as a risk assessment consultant during the 1990s.

He received them from industry friends seeking his expert opinion.

A group of BP executives were on board the Deepwater Horizon rig celebrating the project's safety record. Meanwhile, far below, the rig was being converted from an exploration well to a production well.

Bea believes that the workers set and then tested a cement seal at the bottom of the well. Then they reduced the pressure in the drill column and attempted to set a second seal below the sea floor. A chemical reaction caused by the setting cement created heat and a gas bubble which destroyed the seal.

Deep beneath the seafloor, methane is in a slushy, crystalline form. Deep sea oil drillers often encounter pockets of methane crystals as they dig into the earth.

As the bubble rose up the drill column from the high-pressure environs of the deep to the less pressurized shallows, it intensified and grew, breaking through various safety barriers, Bea said.

"A small bubble becomes a really big bubble," Bea said. "So the expanding bubble becomes like a cannon shooting the gas into your face."

Up on the rig, the first thing workers noticed was the sea water in the drill column suddenly shooting back at them, rocketing 240 feet in the air, he said. Then, gas surfaced. Then oil.

"What we had learned when I worked as a drill rig labourer was swoosh, boom, run," Bea said. "The swoosh is the gas, boom is the explosion and run is what you better be doing."

The gas flooded into an adjoining room with exposed ignition sources, he said.

"That's where the first explosion happened," said Bea, who worked for Shell Oil in the 1960s during the last big oil well blowout.

"The mud room was next to the quarters where the party was. Then there was a series of explosions that subsequently ignited the oil that was coming from below."

Nine rig crew on the rig floor and two engineers died.