Near the height of the war against the Islamic State in Syria, a sudden riot of explosions rocked the country’s largest dam, a towering, 18-story structure on the Euphrates River that held back a 25-mile-long reservoir above a valley where hundreds of thousands of people lived.

The Tabqa Dam was a strategic linchpin and the Islamic State controlled it. The explosions on March 26, 2017, knocked dam workers to the ground and everything went dark. Witnesses say one bomb punched down five floors. A fire spread, and crucial equipment failed. The mighty flow of the Euphrates River suddenly had no way through, the reservoir began to rise, and local authorities used loudspeakers to warn people downstream to flee.

The Islamic State, the Syrian government and Russia blamed the United States, but the dam was on the U.S. military’s “no-strike list” of protected civilian sites and the commander of the U.S. offensive at the time, then-Lt. Gen. Stephen J. Townsend, said allegations of U.S. involvement were based on “crazy reporting.”

“The Tabqa Dam is not a coalition target,” he declared emphatically two days after the blasts.

In fact, members of a top secret U.S. Special Operations unit called Task Force 9 had struck the dam using some of the largest conventional bombs in the U.S. arsenal, including at least one BLU-109 bunker-buster bomb designed to destroy thick concrete structures, according to two former senior officials. And they had done it despite a military report warning not to bomb the dam, because the damage could cause a flood that might kill tens of thousands of civilians.Given the dam’s protected status, the decision to strike it would normally have been made high up the chain of command. But the former officials said the task force used a procedural shortcut reserved for emergencies, allowing it to launch the attack without clearance.