America's National Security Agency may hold crucial evidence about one of the greatest unsolved mysteries of the Cold War - the cause of the 1961 plane crash which killed United Nations Secretary-General Dag Hammarskjold, a commission of prominent jurists says. Widely considered the U.N.'s most effective chief, Hammarskjold died as he was attempting to bring peace to the newly independent Congo. It's long been rumored that his DC-6 plane was shot down, and an independent commission set up to evaluate new evidence surrounding his death on [September 9] recommended a fresh investigation - citing radio intercepts held by the NSA as the possible key to solving the case. Hammerskjold's aircraft went down on the night of Sept. 17, 1961, smashing into a forested area just short of Ndola Airport in modern-day Zambia. A host of hard-to-answer questions about the crash have led to a glut of conspiracy theories. Among them: Why did it take 15 hours to find the wreckage, just a few miles from the airport? Why did Hammarskjold's bodyguard, who survived the crash for a few days, say that the plane blew up"? Why did witnesses report seeing sparks, flashes, or even another plane? Hammarskjold was flying into a war zone infested with mercenaries and riven by Cold War tension. Foreign multinationals coveted [Congo's] vast mineral wealth and the country was challenged by a Western-backed insurgency in Katanga, which hosted mining interests belonging to United States, Britain, and Belgium.