The Defense Department released the name of a soldier who went missing last week after a Black Hawk helicopter crashed off the coast of Yemen, the most recent in a string of military aviation and naval mishaps.

Staff Sgt. Emil Rivera-Lopez is still listed as "whereabouts unknown" and the Aug. 25 crash about 20 miles off the Yemen coast is still being investigated, the department said. Five of his fellow crew members were rescued.

The helicopter and crew were part of elite special operations forces conducting a training mission over the ocean, the Washington Post reported. The U.S. has waged special operations missions in the country, which is embroiled in a civil war that has drawn in Saudi Arabia to fight Iranian-backed rebel forces.

Rivera-Lopez was part of the military's Operation Inherent Resolve mission, which is aimed at defeating the Islamic State group in Iraq and Syria.

A Navy SEAL, Chief Special Warfare Operator Ryan Owens, was killed during a raid in Yemen against al Qaeda in January, sparking controversy during the early days of President Trump's administration.

The Black Hawk crash also follows a string of deadly aviation and naval mishaps across the services, including two Navy ship collisions that killed 17 sailors, two Marine Corps aircraft crashes that left 19 troops dead, and an Army Black Hawk crash into the ocean around Hawaii that killed five.