WASHINGTON — Investigators on Tuesday homed in on whether an uncommon sequence of events, including a decision to remove heavy drilling lubricants early from a pipeline, may have triggered the sudden surge of natural gas that led to the explosion and sinking of the Deepwater Horizon offshore oil rig.

What triggered the explosion that sank the rig and started the flow of crude oil is critical to understanding not only whom to blame for what could become the worst crude spill in U.S. history, but also whether much tougher limitations should be placed on what oil companies can and cannot do when drilling in deep water.

Anthony Gervaso, the engineer aboard a supply ship that was near the rig when it exploded, told a Coast Guard inquiry in Kenner, La., that he’d learned from his captain that rig workers pulled from the water had said they’d just start removing the drilling lubricant from the well when gas shot up the pipe and exploded.

Tim Probert, an executive with Halliburton, the subcontractor responsible for placing a cement plug in the well, told senators in Washington that the dense drilling fluid had been pulled from the drilling tube and replaced with much lighter seawater before a cement plug was set to block gas and oil from coming up the pipeline.

Normally, the procedure would have been to place the plug and then switch out the drilling fluid for seawater. He said the decision to reverse the process came at the instigation of BP, the well’s owner.