BP had workers on the doomed Deepwater Horizon rig who could have prevented the missteps that led to the massive Gulf of Mexico oil spill, but they were not consulted, the White House oil spill commission said on Thursday.

In an expanded report on the causes of the BP drilling disaster that killed 11 workers and ravaged the U.S. Gulf coast last summer, the commission released new details about the events that preceded the BP accident.

The commission’s investigators said BP workers failed to ask a knowledgeable company engineer who was visiting the rig about unexpected results from a critical negative pressure test on the rig.

“If anyone had consulted him or any other shore-based engineer, the blowout might never have happened,” the commission said in a statement.