An air defense system that protected a U.S. coalition base in Syria was “not fully operational” when a suspected Iranian drone hit the base on Thursday, killing a U.S. contractor and wounding six Americans.

Why the system struggled or if it would have made a difference in the drone strike if it had been working properly is unknown. U.S. officials are investigating the matter, military officials told The New York Times. Other defense systems at the U.S. base near Hasakah, Syria, were functioning normally but did not stop the attack.

The United States carried out a series of retaliatory airstrikes on facilities in Syria belonging to Iran’s Islamic Revolutionary Guards Corps (IRGC), which supports militant groups in the region with training and supplies. The U.S. airstrikes hit a munition warehouse, a control building, and an intelligence-collection site. Eight militants died in the strikes, according to a group that tracks conflict in Syria.

The Pentagon revealed the attack on the U.S.-coalition base and the airstrikes against IRGC-backed facilities on Thursday evening.

“At the direction of President Biden, I authorized U.S. Central Command forces to conduct precision airstrikes tonight in eastern Syria against facilities used by groups affiliated with Iran’s Islamic Revolutionary Guards Corps (IRGC),” Defense Secretary Lloyd Austin said in a statement. “The airstrikes were conducted in response to today’s attack as well as a series of recent attacks against Coalition forces in Syria by groups affiliated with the IRGC.”

“As President Biden has made clear, we will take all necessary measures to defend our people and will always respond at a time and place of our choosing,” he added. “No group will strike our troops with impunity.”

Militants responded to the U.S. airstrikes with a missile attack on a U.S.-coalition base near Al-Omar oil field in Syria on Friday. No injuries were reported in the attack.

The spate of violence comes at a time when Iran has grown increasingly influential and aggressive in the region. Iran has been aided in its rise by China. Beijing brokered a peace agreement between Iran and Saudi Arabia earlier this year.

Iran also may receive an air defense system from Russia, which Iran is backing in its war against Ukraine. Iran has been supplying lethal aid to Russia, including drones.

U.S. forces in Syria are on high alert for attacks from Iran-backed groups. The U.S. military estimates that Iran-backed terror groups have carried out 78 attacks on U.S. troops in the country since 2021.