Three U.S. Army soldiers were found dead after their armored recovery vehicle sank in a Lithuanian peat bog during a training mission on March 25.

A fourth soldier remains missing.

The soldiers, from the 1st Brigade Combat Team, 3rd Infantry Division, were deployed for Operation Atlantic Resolve and stationed at Fort Stewart, Georgia.

Recovery efforts took six days, involving U.S. and Lithuanian forces using helicopters, drones, and heavy equipment, NBC News has reported.

An investigation is underway to determine the cause of the accident.

Written by B.C. Begley