The location of the largest dam in Syria was on a US military "no-strike" list because of the risk of flooding, according to The New York Times.
Task Force 9's responsible unit used some of the largest conventional bombs in its arsenal, including one that weighed 2,000 pounds, according to two former senior officials.
According to The Times, Task Force 9 struck the 18-story Taqba dam in March of last year despite a US military report that indicated that a strike on the dam could cause flooding that could kill tens of thousands who live nearby.
At the time of the strike, the region in Syria was occupied by the group.
According to The Times, the US was blamed for the strike by Russia and the Syrian government. Then-Lt. Gen. Stephen J. Townsend dismissed the charge as crazy reporting.
The Tabqa Dam is not a coalition target, according to Townsend.
US Central Command, the US military command center that oversees combat operations in the Middle East, could not immediately be reached for comment, but a spokesman for the agency told The Times that only the operating towers were targeted.
A major flood was avoided thanks to a mix of luck and a team of local engineers.
Syrian engineers on the scene told The Times that one of the bombs dropped remained unexploded on the fifth floor of the complex, and they were able to safely dispose of it.
The two former drone operators said that critical equipment for the operation of the dam was destroyed, and that the dam's reservoir rose 50 feet, nearly spilling, and forcing northern dams in Turkey to temporarily pause their water flow.
According to the report, engineers were able to restore the dam's function after a ceasefire was declared between warring sides in Syria.
The dam's former director told The Times that the destruction would have been unimaginable. The number of Syrians who have died in the war would have exceeded the number of casualties.