Three U.S. Army soldiers went missing in Lithuania during a training exercise, and their bodies have been found.

Tuesday morning the soldiers went missing after their M88A2 Hercules armored recovery vehicle became submerged in a peat bog during a mission to repair and tow an immobilized tactical vehicle.

The soldiers were all from the 1st Brigade Combat Team, 3rd Infantry Division. The identities of the soldiers are being withheld pending family notification, the U.S. Army Europe and Africa Public Affairs Office said.

Military officials said Monday that a fourth soldier remains missing. Search and recovery operations continue for the remaining fourth soldier.

Military officials said the soldiers had been deployed to Lithuania in support of Operation Atlantic Resolve, and they were permanently stationed at Fort Stewart, Georgia. Law enforcement and Lithuanian armed forces worked alongside them during last week’s training exercise.

Early Monday their 63-ton armored vehicle was removed from the peat bog after an arduous six-day effort.

According to the U.S. Army, hundreds of soldiers and law enforcement personnel, American and Lithuanian, scoured the swampy terrain and thick forest to find the missing soldiers.

Specialized equipment was required for the recovery effort to stabilize the soft ground in the area. The effort included fixed-wing aircraft and uncrewed aerial systems, Lithuanian military helicopters, as well as excavators, sluice and slurry pumps and several hundred tons of gravel and earth.

“The soldiers we have lost in this tragedy were not just Soldiers-they were a part of our family. Our hearts are heavy with a sorrow that echoes across the whole Marne Division, both forward and at home,” Major General Christopher Norrie, 3rd Infantry Division commanding general, said in a statement.

“We stand in grief with the families and loved ones of these extraordinary ‘Dogface Soldiers’ during this unimaginable time. But the search isn’t finished until everyone is home.”

The Lithuanian authorities and the U.S. Army are investigating the cause of the accident.

Lithuania, a former Soviet republic, has been a member of NATO since 2004. It has hosted hundred of U.S. troops as part of Operation Atlantic Resolve since 2014, which began after Russia seized Crimea from Ukraine.