An MQ-1B Predator drone crashed in 2017 after its crew lost contact with the remotely piloted aircraft, a report released Monday said — but the reason why remains a mystery.

The Predator, which was from the 432nd Wing at Creech Air Force Base in Nevada, crashed Sept. 4, 2017, in an unidentified part of U.S. Central Command’s area of responsibility, which includes the Middle East and parts of North Africa and Central and Southwest Asia.

The accident investigation board report, which was completed Dec. 17, said the Predator was about 16 hours into a combat support mission when the crew at Creech lost the link, leading to its crash.

The satellite link was lost about one minute after the mishap crew took over from the previous crew. They began troubleshooting with maintainers to re-establish contact, and contacted radar facilities and a nearby fighter aircraft to try to find the Predator, without success.

The Air Force never found the site where the $4.1 million Predator crashed, and its wreckage was not recovered. There were no known injuries, or known damage to other government or private property.

Investigators found no sign of maintenance problems or flawed inspections, and the weather was clear. The pilot and sensor operator were also current and qualified, the report said, and human factors did not play a part in the crash. The Predator had been operating normally and flying at 13,000 feet before the link was lost, the report said.

Another Predator also crashed while on deployment in February 2016 after a datalink failure.