OTTAWA (AP) _ Canada has closed the books on its worst air disaster, concluding after three years of investigation that the crash that killed 256 Americans in Newfoundland is a mystery never likely to be solved.

''There is nothing in all these deliberations which holds out any reasonable prospect of any more success than that achieved to date,'' former Supreme Court Justice Willard Estey concluded in his review released Friday.

''No inquiry is going to bring the deceased back,'' Estey said. ''Should we keep this book open forever?''

Estey was appointed to review the case after an earlier investigation board split sharply on the cause of the crash.

He said the evidence did not support the Canadian Aviation Safety Board's finding that ice on the wings of the Arrow Air charter DC-8 was the probable cause of the Dec. 12, 1985, crash at Gander, Newfoundland.

But he likewise found no evidence to support the minority report, signed by four of the board's nine members in December, that an explosion or fire before impact probably caused the crash.

''It should be no surprise to discover that occasions do arise when no explanation can be found for an event in which the efforts of mankind are overwhelmed by the forces of nature,'' Estey wrote.

Les Filotas, one of the board members who signed the minority report, said circumstances still exist to be investigated ''outside the four corners'' examined by Estey.

''A labyrinth has many more than four corners,'' he said. ''It's very unfortunate that it's going to be closed off.''

Transportation Minister Benoit Bouchard, who appointed Estey to review the persistently controversial investigation in February, said he accepted the recommendation that no further investigation is warranted.

''No further physical evidence can e obtained. It is very unlikely that any analytical approach will be uncovered which would expose the cause,'' Estey wrote.

The investigation was hampered by a cockpit voice recorder that didn't work and a primitive flight recorder that tracked only four instruments.

Estey said at a news conference that it would be ''unproductive and sad'' to reopen the investigation nearly four years after the worst aviation disaster ever in Canada.

''You'll have the same lonely result,'' Estey said.

The crash killed all 248 U.S. servicemen returning to Fort Campbell, Ky., from peace-keeping duties in the Middle East, along with eight American flight crew members.

The review found little justification for the either of the aviation board's conclusions.

''There is nothing in the evidence or the material assembled by the board or in any of the studies prepared by or for the board which would indicate that any further inquiry into this accident almost four years later would determine the cause or causes with any greater certainty,'' the review concluded.

The review noted the references throughout the investigation to possible terrorist involvement as a cause.

''Surmise and speculation inside and outside these proceedings abound, factual evidence does not,'' the review said. ''Nothing indicates any hope of uncovering explanations of this accident in those areas where another inquiry to be undertaken by Canada.''

Estey said he did not find any oversight in the board's work that might have led to a concrete finding of what caused the crash.

He also discounted the charges that the investigation by the safety board, plagued throughout by internal splits, was a coverup.

''There's no evidence of coverup, and I wasn't appointed to cover anything up,'' Estey said.

Estey said the families of those who died in the crash would deserve a continued investigation if reasonable hope of finding its cause existed.

But he also said that the relatives would not be served by endlessly poring over the same material.

Estey reviewed evidence accumulated by the safety board and did not hold new hearings or consider new testimony.

The aviation safety board is being replaced by an agency that will be responsible for all types of transportation, along the lines of the National Transportation Safety Board in the United States.

Kenneth Thorneycroft, the board's chairman, called the review ''very valid, very forceful.''