Carter's remarks came as his top general in Afghanistan, John Campbell, acknowledged the incident was a mistake.
"The Department of Defense deeply regrets the loss of innocent lives that resulted from this tragic event," Carter said in a statement released as he visited Rome on a five-day European tour.
"The investigation into how this could have happened is continuing, and we are fully supporting NATO and Afghanistan's concurrent investigations."
Medecins Sans Frontieres, the medical charity that was operating in the hospital when it was struck, has called the incident a war crime and has pulled out of Kunduz in the aftermath of the attack.
"The US military takes the greatest care in our operations to prevent the loss of innocent life, and when we make mistakes, we own up to them. That's exactly what we're doing right now," Carter said.


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