ON THE GULF OF MEXICO -- 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.

The timeline described in the documents provides the most detailed account of what may have caused the April 20 blast.

Portions of the interviews, two written and one taped, were read to an Associated Press reporter Friday 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.

As the workers removed pressure from the drilling column and introduced heat to set the cement seal around the wellhead, the chemical reaction created a gas bubble and the cement around the pipe destabilized, according to the interviews.

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 the rig's various safety barriers, the interviews said.

Then, the gas surfaced. Then the oil.

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

On Friday a BP-chartered vessel lowered a 100-ton concrete-and-steel vault onto the ruptured well in a delicate and unprecedented attempt to stop most of the gushing crude.