Workers aboard the Deepwater Horizon drilling rig briefly regained control of BP PLC's ruptured oil well on the night of April 20 and may have come far closer than previously believed to averting the disastrous explosion.

The finding, somewhat buried in an internal investigation report issued by BP this week, puts a new, tragic twist on the disaster, which killed 11 workers and set off the worst offshore oil spill in U.S. history.

Contrary to what most oil industry experts thought based on testimony in government hearings, not only did the crew manage to activate the blowout preventer—the huge set of valves designed to shut off the flow of gas in an emergency—but the preventer worked. Unfortunately, workers only triggered it after gas had blown past its valves.

Then, as the gas already in the pipe raced upward toward them, workers decided to divert the flow through a system aboard the rig, rather than over the side, giving the gas a chance to envelop the rig and ignite.

If workers had either realized the problem with the incoming gas moments sooner or steered the flow of the gas differently, the gas might never have reached the rig floor, the report finds.

They "were just so close to this being the topic of a training video," instead of a disaster, said Dan Pickering, co-president of Tudor Pickering Holt & Co., an energy-focused investment bank in Houston.