Head of the United States Central Command Gen. Kenneth F. McKenzie Jr. announced Friday that it is unlikely any ISIS-K members were killed in a Kabul drone strike on August 29, which led to multiple civilian casualties.

"We now assess that it is unlikely that the vehicle and those who died were associated with ISIS-K or a direct threat to US forces," McKenzie said of the airstrike at a briefing, following an investigation by the Military.

"This strike was taken in the earnest belief that it would prevent an imminent threat to our forces and the evacuees at the airport, but it was a mistake and I offer my sincere apology," Mckenzie said, adding that he is "fully responsible for this strike and this tragic outcome."

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The drone strike, which was intended to target ISIS-K operatives, resulted in the deaths of an aid worker and up to nine of his family members, including seven children. According to U.S. officials, the strike on the vehicle, formerly believed to have been a threat that included bombs and that was operated by ISIS-K militants, took place after a suicide bombing at Kabul airport in Afghanistan killed 13 U.S. service members and civilians.

"On behalf of the men and women of the Department of Defense, I offer my deepest condolences to surviving family members of those who were killed, including Mr. Ahmadi, and to the staff of Nutrition and Education International, Mr. Ahmadi’s employer," Secretary of Defense Lloyd J. Austin III said in a lengthy statement on the investigation's findings. "We now know that there was no connection between Mr. Ahmadi and ISIS-Khorasan, that his activities on that day were completely harmless and not at all related to the imminent threat we believed we faced, and that Mr. Ahmadi was just as innocent a victim as were the others tragically killed.

"We apologize, and we will endeavor to learn from this horrible mistake," Austin added, saying that officials "will scrutinize not only what we decided to do -- and not do -- on the 29th of August, but also how we investigated those outcomes." 

"We owe that to the victims and their loved ones, to the American people and to ourselves," Austin said.

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The drone strike, as classified by Mckenzie as a "tragic mistake," was previously hailed as a "righteous strike" by Gen. Mark Milley, Chairman of the Joint Chiefs of Staff.

"In a dynamic high threat environment, the commanders on the ground had appropriate authority and had reasonable certainty that the target was valid but after deeper post strike analysis our conclusion is that innocent civilians were killed," Milley said Friday. "This is a horrible tragedy of war and it's heart-wrenching and we are committed to being fully transparent about this incident."

White House Press Secretary Jen Psaki also described the strike as a success of new "over the horizon" capabilities in late August.

"What we've seen over the past week is that our over the horizon capacity can work and has worked in going after ISIS targets and killing people who went after our troops," Psaki said at the time. "So that's where our resources and our focus is going to be on at this point in time."

Fox News' Jennifer Griffin contributed to this article.