A wounded staff member of Doctors Without Borders and a survivor of the U.S. airstrikes on their hospital in Kunduz, receives treatment at an emergency hospital in Kabul.

United States Defense Secretary Ashton Carter on Tuesday said the Pentagon “deeply regrets” the deaths of 22 people who died in a U.S. air strike on a hospital in the Afghan city of Kunduz.

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, also known as Doctors Without Borders and 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 U.S. 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.