The U.S. military currently is investigating whether the Syrian troops it supposedly bombed on Saturday were, in fact, former prisoners turned into makeshift conscript soldiers for Syrian President Bashar al Assad. That’s according to two U.S. defense officials who spoke with The Daily Beast on condition of anonymity.

It is the first explanation the Pentagon has put forth for why coalition warplanes mistakenly attacked Syrian forces in a strike that was supposed to target the self-proclaimed Islamic State. But it is only one of several theories being examined as investigators try to find out why the strikes, which killed at least 62 people and injured 100 more, went awry.

“That is where we are right now,” one of the officials explained to The Daily Beast. “But that could change.”

The errant airstrikes do not seem to be the result of mistaken coordinates, officials believe. The U.S.-led coalition hit the intended target and it does not appear ISIS or anyone else misled the coalition toward that target site, the officials told The Daily Beast.

“We messed up,” one of the officials concluded.

But officials have yet to explain how they chose the purported ISIS target, particularly one so close to an airbase in the eastern city of Deir el-Zour long under Syrian regime control. Nor could they say why two days of watching the site did not alert them that ISIS was in fact not based there.

Army Gen. Joseph Votel, the head of U.S. Central Command, which is in charge of U.S. operations in the Middle East, is expected to appoint a one-star general Monday to investigate the strike, two defense officials told The Daily Beast.

Turning prisoners, teachers, and others into conscripts is a practice the Syrian regime has employed for at least two years as it has faced increasingly depleted forces. But this is the first time the U.S. military appeared to have mistaken them for ISIS militants.

“This is an actually a regular aspect of the Syrian regime fighting force. The actual Syrian Arab Army is so degraded it doesn’t exist and depends on conscripts,” Jennifer Cafarella, a Syria analyst at the Washington-based institute for the Study of War explained to The Daily Beast. “I am not sure even the Russians know where all these irregular forces are based.”

Such forces would likely not being wearing regime uniforms or forming like a traditional military.

That’s a difference American intelligence assets should have picked that up before the strike, U.S. officials concede. The U.S. military has repeatedly boasted about its precision in—and cautionary approach toward—launching strikes.

The U.S. military was operating two A-10s that were part of Saturday’s strike. In addition, Denmark and Australia, which have expressed remorse for the strike, were also part of the operation. The U.S.-coalition began the operation when Russians officials notified the coalition that it was striking regime forces, leading the coalition to halt the mission midway.

The U.S. is very familiar with Deir el-Zour. It has conducted scores of precise strikes around city, targeting ISIS and should know the city well. The U.S. has conducted, on average, nearly 20 strikes a week around Deir el-Zour, a third defense official explained.

“All of those were valid ISIS targets,” the official said.

Most notably, the U.S. conducted a raid in May 2015 in the area that killed a top ISIS financier, Abu Sayyaf, and captured his wife, Umm Sayyaf, who was believed to have held American hostage Kayla Mueller. ISIS has repeatedly tried to advance on Deir el-Zour and increasingly the Syrian regime forces stopping their advance have been on the defensive.

The effects of the strike, which the Syrian regime has called a “blatant aggression” continued Monday as an already fragile week-long ceasefire all but collapsed. Rebels told Reuters Monday that the ceasefire never really took hold and its fighters plan to resume operations.

Meanwhile, the Syrian Army declared Monday that the pause in hostilities was over, saying in a statement according to the state-owned SANA that “armed terrorist groups took advantage of the declared ceasefire.”