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The carnage was far greater than the famed invasion on Utah Beach, and yet full details about the attack on American vessels off the coast of England during WWII weren’t made public for decades.

In commemoration of the 70th anniversary of Exercise Tiger — a D-Day rehearsal that left 749 soldiers and sailors dead when German torpedo vessels sunk two U.S. ships — an unmanned sub was sent to take the first high-definition sonar images of the wreckage, Hydroid, Inc., the company that surveyed the wreckage, announced Tuesday.

This 2014 sonar image released by Hydroid, Inc., shows wreckage of the bow of an American ship that sank to the ocean floor off the coast of England during World War II, according to the company that surveyed the wreckage to mark the disaster's 70th anniversary. The torpedo attack by German forces on U.S. landing ships, rehearsing during Exercise Tiger for the D-Day invasion of France's Normandy coast, claimed the lives of 749 U.S. soldiers and sailors on April 28, 1944.Hydroid, Inc. via AP

The autonomous underwater vehicle traversed 50 meters below the surface of the water to capture the images, and also recovered an “object of interest” that may hold significance to the doomed ships, according to a statement by Hydroid.

Richard "Bungy" Williams, a Regional Manager at Hydroid, told NBC News that further analysis is required to identify the object.

The sub’s mission off the coast of South Devon, England was conducted from March 10-21, in order to gather the data before the April 28 anniversary, Williams said. The images will be donated to the UK National Archives and local memorials, according to Hydroid.

Exercise Tiger was a “tragic but largely unknown military operation,” Tuesday's statement said.

"Losing his ship and all those friends of his on the ship — that’s the main thing on his mind."

Those who lived through the attack were warned not to say a word and the death toll wasn’t announced until nearly two months after the D-Day, according to The Associated Press. Full details were not disclosed until 1974, when records of the torpedo strike were declassified.

“We hope that the data collected on this mission will shed additional light on this tragic event and help bring some closure to the families who lost their loved ones during Exercise Tiger,” Williams said.

Helen Sadlon, 83, wife of Exercise Tiger survivor, Steven Sadlon, 90, said her husband "would be elated to hear that they were able to do that.”

Steven Sadlon is in the hospital, but Helen told NBC News that she couldn’t wait to tell him of the expedition that aims to bring attention to the attack 70 years later.

Sadlon was a radio operator aboard the first landing craft that was struck by the Germans the night of April 28, 1944, according to an account he penned, featured in “The Star in the Window: Select Stories of World War II Veterans.”

“Losing his ship and all those friends of his on the ship — that’s the main thing on his mind,” Sadlon said of her husband. She said, even to this day, he talks about the devastation “all the time.”

Sadlon suffered a stroke, but he had planned to travel to England for a commemorative ceremony on Sunday at the Tank Memorial, a site dedicated in 1984 to those lost in Exercise Tiger.

Laurie Bolton, honorary director of the Tank Memorial site, was born eight years after her uncle, Sgt. Louis A. Bolton, perished during Exercise Tiger, she said. His body was never recovered.

“In recent years we had divers down at the wrecks and sonar, but nothing like this,” Bolton told NBC News after seeing the sonar images.

Bolton said the images help clear up some of the mystery surrounding the state of the sunken ships and “complete the story.”

“That is my uncle’s burial place, along with hundreds of other men,” she said.