A $4 million military drone was reported lost earlier this week over Lake Ontario in New York.

An MQ-9 Reaper took off on Tuesday, Nov. 12, from Wheeler Sack Army Airfield at Fort Drum and was operating in approved military training airspace. The drone was in its third hour of flight when it dropped into the eastern side of Lake Ontario. A U.S. Coast Guard vessel and helicopter were deployed to search for the drone, but hasn’t located it.

The mission was a training exercise conducted by the New York Air National Guard’s 174th Attack Wing, based in Syracuse, N.Y. It is unclear what caused the drone to go down.

According to NBC News, Col. Greg Semmel, commander of the 174th Attack Wing said the mission was going as advertised up until control was lost. Weather at the time of take-off was “good enough to fly,” but he was unsure of the weather conditions at the time control of the drone ceased. Semmel added that the search was called off for bad weather, but would resume this week.

The Reaper is designed for medium-to-high altitude flights, long endurance and acts as an intelligence, surveillance and reconnaissance asset. It uses sensors to provide real-time data to commanders and intelligence specialists at all levels.

This particular drone model has been a mainstay for U.S. surveillance and remote attacks in Afghanistan, Pakistan and elsewhere. It has a 66-foot wingspan and can cruise at 230 mph with a range of about 1, 150 miles and a ceiling of 50,000 feet.

All other flights from the 174th Attack Wing are on hold as the U.S. Air Force takes control of the investigation.