The US Navy man in charge of the atomic bomb dropped by the United States on Nagasaki at the end of World War II has died of organ failure at the age of 93, a family spokesman announced.

Retired vice admiral Frederick Ashworth, who served as weaponeer aboard a B-29 bomber dubbed "Bockscar" and responsible for the technical performance of the 4.5-ton bomb known as "Fat Man," passed away in Phoenix, Arizona, on Saturday afternoon, following a series of unsuccessful heart surgeries, said Glen Smith, a godson of the late admiral.

Contemporaries remember Ashworth as a key player in the Manhattan Project, a supersecret US government initiative launched in the middle of World War II to equip the United States with a nuclear arsenal.

The heart of the project, which at its peak employed over 130,000 people and cost a total of nearly two billion dollars, was located at Los Alamos National Laboratory in New Mexico, where Ashworth, at the time a Navy commander, held a highly-classified job.

"I think his title was chief operating officer, and he actually helped with the work on the bomb in Los Alamos," Smith said.

After the world's first nuclear test in the New Mexican desert outside of Alamogordo on July 1945, Ashworth was dispatched to Tinian Island in the Pacific and put in charge of the so-called "Destination Team", a group of 54 scientists, engineers and Navy weapons specialists whose job was to assemble the bombs "Little Boy" and "Fat Man" and keep them ready for delivery.

"Little Man" was dropped on Hiroshima from aboard the "Enola Gay," where Captain William Parsons, a close Ashworth associate from the Manhattan Project, served as a weaponeer.

Smith said that Ashworth's job aboard "Bockscar" was "to pull the safety plug and install the actual plugs that were activating the bomb before it was lowered out of the plane."

After the war, Ashworth took active part in Operation Crossroads designed to gauge the effectiveness of various types of nuclear attacks on naval ships.