U.S. forces have borrowed a strategy from Israel known as the “knock on the roof” in an apparent effort to minimize civilian casualties in the fight against the Islamic State.

As ISIS continues to suffer losses in Syria and Iraq, its forces have retreated to population centers in order to blend into the civilian population and use them as human shields against strikes from the U.S. and other Operation Inherent Resolve partners. U.S. Air Force Maj. Gen. Peter Gersten told reporters Tuesday the U.S. engaged in its first knock operation on an ISIS cash hub around four weeks ago.

“We have to measure every weapon and every effect we bring to the battle space,” said Gersten, explaining how Operation Inherent Resolve forces go about trying to prevent civilian casualties during a video briefing with Pentagon reporters.

Approximately four weeks ago, U.S. forces decided to strike a cash hub run by an ISIS “finance emir.” The leader was housing as much as $150 million worth of currency that was being utilized for ISIS operations. Given the U.S. has engaged in several missions targeting ISIS cash hubs in recent months, U.S. forces were interested in striking the target.

After some thorough intelligence gathering, which included engaging in surveillance of the emir’s routine and observation of the personnel coming in and out of the facility, it was discovered a woman and her children were located in the same building the cash was being stored.

In order to mitigate any risk to the woman and her children, U.S. forces decided to borrow the roof knock strategy to give warning to those inside the building. After air-bursting a Hellfire missile over the roof of the building, Gersten said those inside “literally trampled over” the woman in order to get out before the next bomb hit the building itself.

Unfortunately, Gersten said the woman ended up running back into the building just as the next munition hit the building, killing her.

“In this particular event … it ultimately ended up in a civilian casualty,” said Gersten. “Post-weapons release, she actually ran back into the building.”

The knock operation became an often utilized strategy in the 2014 conflict between Israel and Hamas terrorists in the Gaza Strip. In Israel, the military operation became known as Operation Protective Edge.

While firing rockets and other munitions into Israeli civilian population centers, it was discovered Hamas was using human shields in order to prevent Israel Defense Forces (IDF) from striking munitions depots and command centers. Israel began engaging in knock operations prior to striking targets with significant civilian presence. You can watch some examples of the knock operation in action in the video here.

Gersten explained that as much as his forces tried to prevent the woman from being caught in the explosion, there was nothing they could have done once she decided to run back in. Fortunately, the children were able to clear the area and Gersten believes the emir himself was killed, though he could not confirm it.

Immediately after they witnessed the woman being hit in the strike, Gersten said Operation Inherent Resolve conducted a review to see if anything more could have been done. The results of the investigation concluded there was nothing else that could have been done to save the woman, given the unfortunate circumstances. Gersten did not mention why the woman decided to go back in to the building.

Thus far, U.S. forces have only engaged in knock operations once, but Gersten noted they would be willing to use the strategy again, despite the casualty sustained during the first strike.

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