U.S. forces probably did not detect the approach of the Iranian-made drone that killed three American soldiers last week at a remote base in Jordan, and there was no air defense system on site capable of shooting it down, the military’s initial assessment of the attack has found. The early findings, which have not been previously reported, indicate that the drone may have been missed “due to its low flight path,” a U.S. defense official with direct knowledge of the assessment told The Washington Post. Additionally, this person said, the base, known as Tower 22, was not outfitted with weapons that can “kill” aerial threats like drones, and instead relied on electronic warfare systems designed to disable them or disrupt their path to a target.

A common strategy among drone operators and other pilots seeking to minimize or evade radar detection is to fly low to the ground. Another U.S. official affirmed the military’s belief that the drone flew too low to be detected. Both officials, like some others, spoke on the condition of anonymity to discuss a sensitive and ongoing investigation. Defense officials have stressed that assessments can change as investigators learn more.