Three of the four U.S. Army soldiers who went missing during a training mission near Pabradė, Lithuania, last week were found dead on Monday, but the search is ongoing for the fourth soldier, the Army said.

Their identities were not released.

The M88 Hercules armored recovery vehicle the soldiers were in when they went missing was removed from a swamp early Monday morning after six days of work to retrieve it, the Army said.

The soldiers, who are all based in Fort Stewart, Georgia, went missing on Tuesday during a training exercise, the Army said.

On Wednesday, their 63-ton-vehicle was found submerged in about 15 feet of water and "clay-like mud" in a training area, the Army said.

"Most likely, the M88 drove into the swamp," and the vehicle "may have just gone diagonally to the bottom," Lithuanian Defense Minister Dovile Sakaliene told ABC News via phone last week.

The multiday search effort was complicated by the swamp's muddy conditions, officials said.

The Army said last week it brought in assets including "a large capacity slurry pump, cranes, more than 30 tons of gravel, and subject matter experts."

"The Polish Armed Forces have also volunteered a unit of military engineers, which is bringing in an additional water pump, tracked recovery vehicles, other additional equipment and supplies needed along with 150 personnel," the Army said.

On Saturday, a U.S. Navy dive team arrived at the site, joining Lithuanian divers, the Army said.