Scores of soldiers and rescue workers are heading to Lithuania to assist in the recovery efforts of four U.S. Army soldiers whose vehicle has been submerged in a swamp for more than five days.

The soldiers, all part of the 1st Brigade, 3rd Infantry Division, based at Fort Stewart in Georgia, went missing in the early hours of March 25 while on a training mission to recover another Army vehicle.

The incident happened in a training area near Pabradė, a city in eastern Lithuania close to the Belarus border. The vehicle carrying the four soldiers was discovered the next day, buried under a thick layer of mud and water.

Recovery efforts have been complicated by the muddy swampland and massive 70-ton weight of the missing soldiers’ M88 Hercules armored vehicle.

U.S. soldiers attend a Holy Mass at the Cathedral Basilica in Vilnius, Lithuania, on Sunday, for the four U.S. soldiers who went missing.

Mindaugas Kulbis/AP

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Mindaugas Kulbis/AP

In an update, the Army said the vehicle continued to sink into the bog and was about 13 feet below the water’s surface and stuck in more than 6 feet of mud, which U.S. officials have described as clay-like.

“It is highly complex trying to get to the vehicle itself with the terrain out here and where the M88 is sitting in a bog swamp-like area, below the waterline,” Brig. Gen. John Lloyd, commander of the U.S. Army Corps of Engineers North Atlantic Division, said in a statement Sunday.

He added: “So not only are we dealing with the terrain, a lot of mud that is over top of the vehicle, but also the fact that it’s 70 tons that we’re trying to recover out of a swamp or bog.”

A specialized U.S. Navy dive team, who arrived on site and made an initial dive on Saturday, also had a difficult time navigating the swamp. “Last night, divers were in the water trying to get to the vehicle. We were unable to because of the amount of mud,” Lloyd said.

But on Sunday, the U.S. military said the dive team managed to successfully attach a line to one hoist point on the submerged vehicle. The goal is to hook up a series of hoists in order to pull the vehicle out of the mud.

“This is the first big step towards successfully recovering the vehicle and bringing our Soldiers home,” the U.S. Army Europe and Africa said in a statement. “We expect that process to take some time, as the amount of pressure and suction from the mud will take significant power to overcome.”

Military personnel work at the site of a rescue operation for missing U.S. soldiers at Pabradė training ground, in Lithuania, on Friday.

Petras Malukas/AFP via Getty Images

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Petras Malukas/AFP via Getty Images

Over the past week, rescue crews have been working to remove water and mud from the site using various draining, digging and dredging techniques. The Army said the process has been slow and challenging due to “ground water seepage.” The terrain has been unable to support the heavy equipment required to extract the vehicle.

More than 200 personnel have been involved in the recovery effort, including American and Lithuanian soldiers, Lithuanian authorities, and 55 engineers from the Polish Armed Forces, according to the U.S. military.

The families of the four missing soldiers have been notified and the U.S. military said it is continuing to update the families on the status of search efforts.

“This tragic situation weighs heavily on all of us,” U.S. Army Maj. Gen. Curtis Taylor, the commanding general of 1st Armored Division, said last week in a statement. “We want everyone to know, we will not stop until our Soldiers are found.”

Lithuanian defense officials prayed for the recovery efforts at a Holy Mass on Sunday, according to the country’s defense ministry.

“Shoulder to shoulder, we stand together until we find missing soldiers,” Lithuanian Defense Minister Dovilė Šakalienė said on social media on Sunday.