WASHINGTON (AP) — The two Iowa National Guard members killed in a weekend attack that the U.S. military blamed on the Islamic State group in Syria were identified Monday.

The U.S. Army named them as Sgt. Edgar Brian Torres-Tovar, 25, of Des Moines, and Sgt. William Nathaniel Howard, 29, of Marshalltown.

Iowa Gov. Kim Reynolds ordered all flags in Iowa to fly at half-staff in their honor, saying that, “We are grateful for their service and deeply mourn their loss.”

The Pentagon’s chief spokesman, Sean Parnell, has said a civilian working as a U.S. interpreter also was killed. Three other Guard members were wounded in the attack, with two of them in stable condition and the other in good condition.

This attack poses a significant challenge to the U.S.-Syria relationship which has seen some cooperation since the overthrow of Bashar Assad last year. Hundreds of American troops remain deployed in eastern Syria as part of a coalition to combat IS.

The shooting occurred in the Syrian desert near the historic city of Palmyra, injuring local security forces and resulting in the death of the gunman. It was revealed that the assailant was a recent recruit in Syria’s internal security forces, prompting concerns over potential IS affiliation.

President Donald Trump vowed serious retaliation for the attack and noted that Syrian President Ahmed al-Sharaa was in shock, emphasizing that Syria and the U.S. are fighting together against IS.