An Israeli Air Force drone crashed into the sea after suffering a major malfunction during a flight Tuesday afternoon off the coast of Palmachim.

IAF search and rescue forces scrambled to the area of the crash to retrieve the remains of the Hermes 450 aircraft. The cause of the accident is unknown but early assessments point to engine failure.

In July troops preemptively crashed a Hermes drones after discovering a malfunction in its systems. On that occasion the unmanned aircraft was flying near the Egyptian border when its operators noticed the problem. They decided to crash it inside Israeli territory to avoid the risk of the advanced aircraft falling into Egyptian hands.

The Hermes 450, made by Elbit Systems, is a medium-sized drone designed for reconnaissance and surveillance missions that can fly for over 20 hours straight. Each drone costs about $2 million.

In May Israel’s military grounded a fleet of high-altitude Heron-1 UAV surveillance drones after one was downed over the Mediterranean Sea. The military intentionally crashed the unmanned aircraft because of a malfunction in its engine. The failure was discovered during a routine flight off the coast of Netanya.

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.