The Pentagon has released the identities of six US service members killed when a KC-135 refueling aircraft crashed in western Iraq earlier this week. In a statement on Saturday, the Pentagon said preliminary findings indicate the crash was not caused by hostile action. The incident occurred Thursday and increased the number of US personnel killed in the anti-Iran military campaign to at least 13. Another aircraft involved in the operation landed safely, according to officials.

The Pentagon confirmed that the victims were crew members on duty at the time, including three from the US Air Force and three from the Air National Guard. The cause of the crash remains under investigation, though US Central Command stated it was not due to enemy fire or friendly fire. Meanwhile, the Iran-backed coalition “Islamic Resistance in Iraq” claimed responsibility for downing a KC-135 and said it had targeted another aircraft that escaped.

The KC-135 crash marks at least the fourth US military aircraft lost in the ongoing conflict, following earlier friendly-fire incidents over Kuwait.