An internal military investigation has concluded that two civilians were killed in a U.S.-led coalition airstrike against the self-proclaimed Islamic State, two defense officials confirmed to The Daily Beast, marking the first time the U.S. military has acknowledged killing a civilian since the air campaign began nine months ago.

In that time, the U.S.-led coalition has conducted more than 3,500 strikes and either destroyed or damaged more than 6,000 targets, according to the Defense Department. Previously, the U.S. military had said it had no evidence that a civilian had ever been killed in the air campaign against ISIS, a claim that even military officials privately acknowledged was hard to believe, given the high odds of unintended mistakes.

Indeed, with no U.S. soldiers on the ground to assess the damage inflicted by airstrikes, the coalition’s air campaign is built on U.S. intelligence collected from drones, satellites, and reconnaissance aircraft, as well as information from local troops.

The findings of two civilian deaths come as one human rights group has alleged that coalition airstrikes on April 30 in the Syrian village of Bir Mahli killed as many as 64 civilians. Bir Mahli sits in Aleppo Province, east of the Euphrates River, and about 30 miles south of the northern city of Kobani. The allegation, which the U.S. military said it has no evidence to corroborate, is the highest civilian death toll accusation leveled at the coalition.

The U.S. military’s claim of no civilian casualties in its campaign against ISIS, coupled with the lack of detailed accounting of what effect the strikes are having, has only underscored the opaque nature of the battle, fought largely from the air with uncertain outcomes.

Officials with U.S. Central Command, which conducted the investigation, could release details of their findings as early as this week, the defense officials said, declining to elaborate on those findings.

CENTCOM officials had previously said that they were investigating allegations of civilian deaths in four cases—two in Iraq and two in Syria—from August 8, 2014, when air campaign began, to around mid-March of this year. At least three of those internal investigations were prompted within the U.S. military, CENTCOM officials have said.

CENTCOM officials declined to comment about the investigation’s findings, saying, in a statement, “We will make additional information about the investigation available when the process is complete.”

But officials stressed that they are committed to reducing civilian deaths.

“No other military works as hard as we do to be precise in the application of our airstrikes. We have significant mitigation measures in place within the targeting process and during the conduct of operations to reduce the potential risks of collateral damage and civilian casualties,” CENTCOM spokesman Army Maj. Curt Kellogg said in a statement. “I can assure you that before any mission, every precaution is taken to ensure civilians are not harmed. Our efforts stand in stark contrast to the tactics of [ISIS], who continues to kill, torture, and abuse civilians as well as embed their combatants in civilian areas.”

In Bir Mahli, the Britain-based based Syrian Observatory for Human Rights and the U.S. military offer widely varying accounts of what happened in the April 30 strikes. Shortly after the attack, the U.S. military said it killed 50 ISIS fighters and struck ISIS fighting positions in a largely abandoned village. Since then, two defense officials told The Daily Beast that the decision to launch the strike was based, in part, on information from Kurdish forces on the ground that there were no civilians remaining in the small village as well as U.S. intelligence findings that ISIS fighters were operating there.

“We currently have no indication that any civilians were killed in these strikes,” CENTCOM spokesman Kellogg said in a statement. “We have significant mitigation measures in place within the targeting process and during the conduct of operations to reduce the potential risks of collateral damage and civilian casualties. We work extremely hard to be precise in the application of our airstrikes and take all allegations of civilian casualties very seriously.”

But Rami Abdul Rahman, who leads the Observatory, said civilians who still live in village, many of them related to those killed, have provided a list of names of civilian victims, including 31 children and 19 women.

“They were very upset when the U.S. said it killed ISIS. The villagers said, ‘How do you know who you killed?’” Abdel Rahman told The Daily Beast.

But as of Monday, there was no indication that the human rights group had any video or photo evidence to bolster its claims. And like the coalition, such groups often rely on limited information. In all, the Observatory has alleged that coalition airstrikes have killed 132 civilians and more than 2,000 ISIS fighters.

U.S. officials have said they take great care to avoid civilian casualties, and there have even been charged by some local forces on the ground of the U.S. military being overly cautious. But the steady stream of accusations, coupled with the U.S. military’s inability to say with certainly that such claims are false, has only reaffirmed fears that an untold number of civilians have been killed in the fight against ISIS.

U.S. military officials said they investigate any credible claim of civilian casualties, and so far have not received any information that would lead them to believe the strikes killed civilians.

Where civilian casualties were once a key metric of the U.S.-led war in Iraq, during the battle with ISIS civilian casualties has been a number few want to discuss. Congress has not pushed top military leaders when they testify on Capitol Hill about those statistics.

Meanwhile, because the Iraqi government has, at times, depended on airstrikes to wrest control of cities out of ISIS control, they too have not pushed the issue. In Syria, the strikes potentially help President Bashar al-Assad because they target one of his major foes, leading to no major resistance in that country either.

Because of these political and strategic calculations, the care given to civilian avoiding and counting civilian casualties rests largely with the same forces conducting the operations.