WASHINGTON (AP) — The equipment failures blamed for the Gulf oil spill might have been detected if the owners of the Deepwater Horizon continued to have the rig’s drilling equipment verified by independent experts — something federal regulators mistakenly thought was happening offshore.

At least three government drilling engineers assigned to the federal investigation into the spill have acknowledged they had mistakenly assumed these third-party examinations were taking place. The examinations are voluntary under U.S. law and most rigs don’t do them, even though they are more hands-on than routine government inspections.

This lapse in the offshore energy industry’s safety net has emerged as a key finding in the joint federal inquiry by the Coast Guard and Bureau of Ocean Energy Management, Regulation and Enforcement into the largest offshore oil spill in U.S. history.

Now, the Obama administration has ordered every rig in the Gulf of Mexico to subject a central piece of drilling hardware — the blowout preventer — to certification by a third party before any can drill again. It is also investigating the extent of the regulators’ fundamental misunderstandings.