Story highlights Former UN Secretary-General Dag Hammarskjold died along with 15 other passengers

He is remembered as perhaps the United Nations' most effective and boldest executive

Ban Ki-Moon calls for further investigation

United Nations (CNN) The dry, tedious world of the United Nations system might have a major mystery on its hands.

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.

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