UNITED NATIONS (AP) — The head of the latest investigation into the 1961 plane crash that killed U.N. Secretary-General Dag Hammarskjold is reiterating that an external attack may have downed the aircraft — and is urging the U.S., Britain, South Africa and Russia to provide more information to help clear up the mystery.

A former chief justice in Tanzania, Mohamed Chande Othman, said in a 95-page report released Monday that based on all information now available "it appears plausible that an external attack or threat may have been a cause of the crash, whether by way of a direct attack ... or by causing a momentary distraction of the pilots."

But Othman said it hasn't been possible to conclude whether sabotage may have been a cause because of difficulties in obtaining relevant documents in South Africa.

He said it also remains possible the crash resulted from pilot error, "despite the experience of the crew and the otherwise normal conditions that preceded the approach to landing "

Widely considered the U.N.'s most effective secretary-general, Hammarskjold was on a peace mission to Congo and died when his chartered DC-6 went down near Ndola Airport in modern-day Zambia, then the British protectorate of Northern Rhodesia.

Congo won its freedom from Belgium in 1960 and the Swedish diplomat was flying into a war zone infested with mercenaries and riven by Cold War tensions. Multinational companies coveted its vast mineral wealth and the country's government was challenged by a Western-backed insurgency in Katanga province, which hosted mining interests belonging to the U.S., Britain and Belgium.

U.N. Secretary-General Antonio Guterres said in a letter transmitting the report that Othman had reaffirmed the conclusions of his 2017 report to the General Assembly: that it is plausible an external threat or attack caused the crash.

He said Othman had received new information about possible causes in the past two years, including probable intercepts of relevant communications, information on the military capacities of parties for staging a possible attack, and "the presence in the area of foreign paramilitary, including pilots, and intelligence personnel."

But while Othman "has made much progress," Guterres said, "it is equally clear that the work will need to continue with renewed urgency, with a view to establishing the truth of the tragic event."

Othman said he received crash-related photographs and new information on the interception of radio communications, the role of air traffic control in Ndola, other individuals "of interest," other possible aircraft operating in the area, acts of local and foreign authorities, and crash site witnesses.

He stressed that the search for information about the crash has not been "exhaustive," saying the United Kingdom, United States and South Africa especially, but also Russia, have failed to respond "substantively" to queries about possible material in their intelligence, security and defense archives.

Othman said he asked 14 countries to appoint an independent and high-ranking person to perform a comprehensive internal review of intelligence, security and defense archives in those nations.

He commended Belgium, France, Sweden and Zimbabwe for "the depth and volume" of work by their appointees. And he said he was grateful for the work by appointees from Canada, Germany, Portugal and Zambia although there was less potential material they had to cover.

Othman said Angola replied that it was a Portuguese colony at the time and had no access to classified documents. Congo made an appointment and provided an interim report in July, saying the research was underway and access had been granted to all documents and archives, both classified and unclassified, he said.

Othman said South Africa made an appointment last May — 15 months after his request — which he called "a positive step" but he said he had not received any further information.

Britain also made an appointment in May and sent a letter the following month saying it had submitted all relevant documents, he said.

As for the United States, Othman said, an appointment was made in 2018, and in 2019 "I was advised that searches were ongoing, but responses to my substantive queries and requests for specific information were not received."

Regarding Russia, Othman said he was advised it had searched intelligence, security and defense archives but it did not appoint an independent person or provide details. He said he wrote to Russia in March to request "further engagement" on an independent appointee.

Othman said existing information and historical records show South Africa, the United Kingdom and the United States "hold important undisclosed information." They created or received records resulting from interceptions of United Nations and other communications, he said, and they had intelligence, security and defense sources monitoring or involved in events around the crash.

Othman made four recommendations: To continue the investigation, to again urge countries to appoint an independent person to determine whether relevant information about the crash exists, to have the next investigator report at the end whether countries complied, "including an observation as to whether any inference may be drawn as a result of non-compliance," and to ensure the U.N. continues to work to make key documents publicly available online.