Allowing Americans to bring advanced underwater robots into Vietnamese-controlled areas of the South China Sea – equipment that could have military uses for either government – is also diplomatically delicate. Pietruszka said obtaining permits for the latest expedition was “a difficult task for all parties”.

But US-Vietnamese relations have warmed steadily since relations between the two countries normalized in 1995. And for Vietnam, allowing such projects is a way to build further trust with its former enemy, said Le Van Cuong, a retired Vietnamese major general.

“The most outstanding trait of the Vietnamese is a desire to help others,” he added.

Paul Andrew Avolese, whose family refused to be interviewed, was born on June 12, 1932, according to military archival documents. He was from New York and served in the Air Force’s 4133d Bomb Wing in Vietnam.

On July 7, 1967, he and his crew flew with other B-52s from a US base in Guam to bomb a target in South Vietnam, documents show. When two of the bombers maneuvered about 65 miles southeast of the then South Vietnamese capital Saigon, they collided and ignited a “ball of fire”. One person on Major Avolese’s plane, Major General William J. Crumm, was the first of several American generals killed in the war.

Credit…US Air Forces

Eight days after the crash, Col. Mitchell A. Cobeaga of the Air Force wrote to Major Avolese’s parents that the exact cause of the collision was unknown. “Every man here at the 4133d Bomb Wing shares your fear for your son,” he added.

Major Avolese, who was 35 years old at the time of the crash, was pronounced dead a few days after the letter was drafted. The US military later classified his remains and those of the five other missing persons as “unrecoverable”. Nevertheless, the investigators pursued possible clues to the debris of the two B-52s for decades.