A U.S. military attack that killed nearly a dozen Iraqi soldier allies was a “mistake,” Defense Secretary Ash Carter admitted on Saturday.

“These kinds of things happen when you’re fighting side by side as we are,” Carter told reporters of Friday’s airstrike near Falluja, which killed an estimated 10 people.

One U.S. military official told CBS that the attack was reportedly made in coordination with Iraqi forces, and that bad weather could have been a factor in the accident. “The folks on the ground who we were communicating with were closer to the targeting area than we understood,” the official said.