(CNN) -- A timeline of events leading to the April 20 explosion on the Deepwater Horizon oil rig in the Gulf of Mexico, according to documents from congressional investigators and testimony from witnesses to the disaster:
Oilfield services contractor Halliburton completes cementing on BP's Mississippi Canyon 252 well. In the hours leading up to midafternoon, crews conduct three positive pressure tests, in which fluid weight is placed on the seal.
BP and Transocean executives argue over changes in the drill plan for the well at a meeting aboard the rig.
An unexpected loss of fluid is observed in the riser pipe, suggesting there are leaks in the blowout preventer, a critical piece of equipment designed to shut down the well in the event of an emergency.
Negative pressure tests, in which crews reduce fluid pressure to test for leaks through the cement or well casing, show unexpected results. One draws 15 barrels of liquid out of the well instead of the expected five, raising more concerns about leaks. In a second, pressure on the rig's "kill line" -- a high-pressure pipe used to cut off the flow of oil -- falls to zero, while rising to 1,400 pounds per square inch in the drill line, indicating a buildup of natural gas.
BP ends testing and proceeds to replace remaining drill fluids with seawater. A BP investigator later suggests this is a "fundamental mistake."
More fluid is reported flowing out of the well than is being pumped in.
Well pump is shut down for a "sheen" test, but the well continues to flow. Drill pipe pressure unexpectedly increases.
Abnormal pressures and more fluid returns are observed. The well pump is abruptly shut down.
Gas surges from the well and up the riser. The supply ship Damon B. Bankston, which is tied up alongside Deepwater Horizon, reports drilling fluid spilling onto its deck and is told to move back 500 meters from the rig. Seconds later, the first explosion occurs.
Deepwater Horizon issues distress call. The order is given to abandon the rig. Eleven of the 126 on board are lost at sea when the platform sinks two days later.