Israel’s military grounded a fleet of high-altitude surveillance drones on Sunday after one was downed over the Mediterranean Sea.

The military said it intentionally crashed the unmanned aircraft late Saturday because of a malfunction. The IAF Heron-1 UAV was downed by the air force after its controllers discovered an engine failure. The failure was discovered during a routine flight over the Mediterranean, off the coast of Netanya.

The IDF’s Spokesman’s Office said the army shot down the drone fearing it would crash into a populated area. Local residents reported seeing helicopters searching the area by the light of flares in an effort to locate remains from the crash.

The military did not say how many aircraft were grounded, but the planes will stay down during an investigation.

The Israeli-made Heron-1 is one of the most advanced UAVs in the world. It can remain airborne for more than two days without requiring refueling, gathering intelligence from high altitudes. Last year, a larger Heron TP drone crashed on a routine flight.