A BP report tersely described the results: “No indication of movement.”

But engineers could not be absolutely sure. Without any way to see into the blowout preventer, engineers had essentially been operating blind, using the rate of oil flow, for example, to deduce the conditions inside.

Help came from Scott Watson, an expert in gamma ray imaging at Los Alamos National Laboratory. Gamma rays, a form of electromagnetic radiation similar to X-rays but higher in energy, might at least penetrate a few inches into the blowout preventer’s thick steel walls. Then engineers might be able to see a device called a wedge lock, which slides into place behind the shear ram to hold it closed.

In mid-May, Mr. Watson ventured to the well site, where robotic submersibles were sent down to the seafloor with cobalt 60, a radioactive isotope that generates gamma rays. The team from Los Alamos was able to get a clear view of only one half of the blind shear ram. But the images showed one wedge lock fully engaged, meaning at least one half of the shear ram had deployed.

“I don’t think anybody who saw the pictures thought it was ambiguous,” Mr. Watson said.

It was a crushing moment.

Engineers realized that all their efforts to revive the blowout preventer had probably never budged the critical component at the machine’s core, the blind shear ram. They had assumed that at some point early on, the blades had tried to close. They had hoped to close them all the way. But now, the gamma ray images showed that at least one blade was fully deployed, and they had run out of options for forcing the other one closed. Continuing to push on the ram’s pistons with more hydraulic fluid would achieve nothing.

The last line of defense was a useless carcass of steel.

False Sense of Security

Barely three weeks before the Deepwater Horizon disaster, President Obama announced that he planned to open vast new tracts of ocean for oil exploration, including environmentally sensitive areas that for decades had been declared off limits by presidents from both parties.

Environmental groups were bitterly disappointed, but Mr. Obama said he had arrived at his decision after more than a year of study by his administration, including a careful weighing of environmental risks. Yet the administration’s examination did not question the oil industry’s confident assertions about its drilling technology. The well-known weaknesses of blowout preventers and blind shear rams simply did not make it onto the administration’s radar, interviews and documents show.