The question is this: Was acclaimed former U.N. Secretary-General Dag Hammarskjold murdered, instead of being a victim of a plane crash accident in 1961?
The case is back in the spotlight after Ban Ki-Moon, the current secretary-general, called for further investigation this week.
Hammarskjold died along with 15 other passengers on September 18, 1961, when their plane went down in a forest.
The Swedish diplomat was en route to broker a truce between rebels and the government, and to unite the Congo in southern Africa.
Hammarskjold, the body's second secretary-general, received the Nobel Peace Prize following his death. He is remembered as perhaps the United Nations' most effective and boldest executive during a time of Cold War tensions.
Though it was generally believed at the time the crash was not suspicious, it didn't take long for conspiracy theorists and others to say that Hammarskjold did not die by accident.
Former U.S. President Harry Truman stoked fears when he told reporters just two days after the crash, "Dag Hammarskjold was on the point of getting something done when they killed him. Notice I said, 'When they killed him.'"
Truman didn't elaborate.
Now, Ban says there's enough new information to question what has so far been determined or assumed about the crash, which occurred in what is now Zambia.
Ban, a long time admirer of Hammarskjold, said an investigation should be done after a special panel he appointed found Hammarskjold could have been targeted by air, or some other form of attack.
The panel found new information, which was assessed as having "moderate probative value" -- enough to further pursue aerial attack or other interference as a hypothesis of the possible cause of the crash.
The three-member panel interviewed new witnesses who claimed they had seen two planes in the sky that night, one of which caught fire before crashing. The report also says the local government in the region included foreign troops that may have had "air capability."
The report did rule out sabotage or hijacking as potential causes of the crash. It said new evidence was found to look at crew fatigue as a possible contributing factor.
To further deepen the mystery, any new investigation would include testimony from former U.S. national security intelligence officers.
Two of them talked with the U.N. panel and said they had listened or read radio intercepts that suggested the secretary-general's plane was attacked. The report also states that the leader's cryptography machine had been designed to allow select intelligence agencies, including the National Security Agency, to listen.
In a statement, Ban said a new investigation would "finally establish the facts." But the U.N. chief needs help from some of the countries that appointed him.
The United States and Britain did not fully comply with requests made by the special U.N. panel appointed by Ban.