The U.S. and Canadian governments bungled the probe of a crash that killed 256 U.S. servicemen and may be concealing a possible terrorist connection, a congressional panel was told yesterday.

Rep. Robin Tallon (D-S.C.) told the House Judiciary subcommittee on crime that federal agencies appear guilty of "ineptness or the best-contrived coverup. . . ever" in the Dec. 12, 1985, crash in Gander, Newfoundland.

The chartered military DC-8 carrying U.S. peacekeeping troops home from the Middle East crashed less than 30 seconds after takeoff. The Canadian investigation decided the probable cause of the crash was ice on the wings.

A subcommittee staff report said there was no evidence that either the United States or Canada had seriously investigated possible terrorism.

"Incredibly, no U.S. government agency, or at least none with official responsibility, demonstrated any determination to find out just what caused the crash," said subcommittee Chairman William J. Hughes (D-N.J.).

"The National Transportation Safety Board and other agencies of the U.S. government chose to sit back and watch as the Canadian efforts became embroiled in controversy," Hughes said. "The NTSB routinely rubber-stamped the Canadian findings."

Eyewitnesses say they saw a bright orange object in the sky at the time of the crash, suggesting that the plane was on fire. An anonymous caller later said Islamic Jihad, a pro-Iranian terrorist group, had put a bomb on the plane.

"When a terrorist organization took credit for this abomination, why was this claim not investigated and a report made to the American people?" asked retired Army Col. Lewis Millett.

The theory of ice on the wings is "preposterous," said L.T. Filotas, a Canadian aeronautical engineer who served on the investigating panel and dissented from the majority's finding.

Tallon and others suggest terrorists might have blown up the plane in connection with the Iran-contra affair. The Reagan administration authorized a shipment of Hawk antiaircraft missiles to Iran less than three weeks before the plane crashed, but the missiles were not the kind the Iranians were expecting and the Iranians felt they had been duped, according to computer messages from former White House aide Oliver L. North. The messages were uncovered by the Iran-contra congressional committees.

Terrorists might have planted an explosive device aboard the plane in retaliation, said Dr. J.D. Phillips, a pathologist whose only son was killed in the crash.

The pilot of an earlier leg of the plane's journey told the Federal Bureau of Investigation he believed that either structural failure or a bomb had caused the crash.

The pilot also told the FBI that an Egyptian Army soldier guarding the plane on the ground in Cairo disappeared from his post several times.

The FBI and the Royal Canadian Mounted Police "found very serious allegations" but that information apparently was not turned over to investigators, Tallon said.