The U.N. General Assembly unanimously adopted a resolution Thursday urging all countries — especially the United States, Britain and South Africa — to release all information on the mysterious 1961 plane crash that killed U.N. Secretary-General Dag Hammarskjold.

An independent panel reviewing new information about the crash said in July that the United States and Britain retain some classified files, and that South Africa had not responded to several requests for information.

The 99-page report puts to rest claims that Hammarskjold was assassinated after surviving the crash but provides new information about a possible aerial attack or interference.

The resolution, which was adopted by consensus, welcomes the progress made by the panel and asks Secretary-General Ban Ki-moon to pursue pending requests for information that its members made.

Ban appealed to all states Wednesday to disclose and declassify all available material, saying he is "personally invested" in fulfilling the U.N.'s duty to establish the facts after so many years.

"A further inquiry or investigation would be necessary to firmly establish the facts," the resolution said.

Widely considered the U.N.'s most effective chief, Hammarskjold, a Swedish diplomat, died as he was attempting to broker a cease-fire in 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 recommended a fresh investigation in September 2013, citing radio intercepts held by the U.S. National Security Agency as the possible key to solving the case.

The three-member panel, appointed by Ban in response to a General Assembly request last December to review new evidence, gave a "moderate" value to information from nine of 12 new eyewitnesses who reported on the final stages of the flight before it crashed near Ndola Airport in modern-day Zambia, then Northern Rhodesia.

The panel's report said the nine eyewitnesses helped establish one or more of the following: there was more than one aircraft in the air as Hammarskjold's aircraft approached the airport, the aircraft were jets, the secretary-general's DC-6 was on fire before it impacted the ground, and his aircraft "was fired upon or otherwise actively engaged by other aircraft present while approaching Ndola."

The panel also gave a "moderate" value to claims by two Americans — Charles Southall, a former U.S. Navy commander, and Paul Abram, a former U.S. Air Force Security Services officer — who either listened to or read a transcript of an intercept of radio transmissions the night of Sept. 17-18, 1961, which they believe was reporting an attack on Hammarskjold's plane that resulted in the crash.

It also cited a report that an alleged agent for Britain's foreign intelligence agency MI6, Neil Ritchie, transported Moise Tschombe, leader of Congo's secessionist Katanga province, to Ndola on Sept. 17, 1961, to meet Hammarskjold. The panel said Britain has refused to release files on British intelligence agency activities in the area.

The panel cited documents from the South African Institute for Maritime Research that refer to "Operation Celeste," purportedly to "remove" Hammarskjold with cooperation from then U.S. CIA director Allen Dulles. The documents noted that the U.N. chief would be in Leopoldville on or about Sept. 12, 1961, the panel said.