An independent review of new information about the mysterious 1961 plane crash that killed U.N. Secretary-General Dag Hammarskjold puts to rest claims that he was assassinated after surviving the crash but provides new information about a possible aerial attack or interference.

U.N. Secretary-General Ban Ki-moon called for a further inquiry or investigation in a letter to the U.N. General Assembly circulated Monday saying some countries believed to have information about the crash didn't provide "a substantive response," didn't respond at all or maintained that documents were classified.

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 General Assembly late last December asked Ban to appoint an independent panel to review new evidence. Its report released Monday 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 three-member panel 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.

The 99-page report states that the United States and Britain retain some classified files and says South Africa has not responded to several requests for information.

Among the new information made available to the panel by the independent commission was 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.

While the report doesn't comment on the possible cause of the crash, the panel said it provides new information about the presence of the British intelligence agency in the area. It added that Britain has refused to release these files.

The panel said it also investigated sabotage as a possible cause of the crash and while there was no evidence of an explosion while the plane was in flight it received several pieces of new information that relate to the possibility that the DC-6 crashed as a result of sabotage.

It cited eight documents in a file that the South African Truth and Reconciliation Commission received from the country's National Intelligence Agency relating to the 1993 assassination of Chris Hani, leader of the South African Communist Party, which purported to be internal correspondence of an organization allegedly engaged in clandestine mercenary activities in and around Congo and elsewhere in the early 1960s.

Documents from the organization, the South African Institute for Maritime Research, refer to an "Operation Celeste," purportedly to "remove" Hammarskjold with cooperation from then U.S. CIA director Allen Dulles, and note that the U.N. secretary-general would be in Leopoldville on or about Sept. 12, 1961, the panel said.

One document refers to a Belgian mining company providing explosives and detonators and another indicates the device failed on takeoff but activated prior to landing and ends "Mission accomplished: satisfactory," it said.

The panel said it has received no response from the South African government to its request for information, records, or references about the existence and activities of the organization that would either negate or corroborate the purported plan.