The Italian transport minister said yesterday an airliner that exploded and crashed without explanation into the sea off Italy last June, killing 81 persons, most likely was shot down by a missile.

The declaration by minister Rino Formica, before a parliamentary committee in Rome, revived the question of what caused the mysterious crash and gave new impetus to months of speculaton about who could have fired such a missile and why over the supposedly peaceful waters of the Tyrrhenian Sea southwest of Naples.

Adding to the mystery were reports in Washington of an unexplained blip on tape recordings of the plane's radar track. U.S. experts, who investigated the crash at the Italian government's invitation, said the blips explained little for certain, although they did indicate something was in the sky near the airliner immediately before it plunged into the sea.

The Italian press has speculated at length on official conjecture that a missile brought down the aircraft, a DC9 of Italy's internal airline Itavia on a June 27 flight from Bologna to Palermo. Possibilities raised have ranged from Libyan-backed terrorists settling a mysterious account to Italy's own Red Brigades revolutionaries making a spectacular blow against Italian society -- to an officially hushed-up collision with an off-course NATO trainer plane.

No terror group has publicly claimed responsibility for the crash and, as far as is known, the American-made jetliner carried no passengers who were obvious assassination targets. But because there were no survivors and the "black box" flight recorder was never recovered, speculation about a missile attack has persisted, fueled by the statements from Italian officials.

Carlo Davanzali, the Itavia president, opened the current round with a letter to Formica on Tuesday saying he is "certain" the plane went down after being hit by a missile, United Press International reported from Rome. In response, Formica told the Italian parliamentarians yesterday that "the missile theory is the strongest," UPI said. He added that investigators have ruled out a structural defect in the airfcraft or collision with another plane, but were unable to say whether the explosion originated inside or outside the fuselage.

U.S. government and industry sources confirmed that the U.S. National Transportation Safety Board, after extensive studies of the radar tapes, concluded that some object other than the DC9 was in the immediate vicinity just before the plane disappeared from the radar screens.

"There was something there that shouldn't have been there," one source said. "There is an ojbect on radar that eventually winds up going through some debris or scatter behind the DC9. But to put an interpretation on it that there was a projectile or missile would be strictly conjecture."

Moreover, there was no indication on the tapes, another source said, that the object was on a flight path that intercepted the DC9 -- a pattern one would expect if it were a missile. "It just showed up," the source said.

According to industry experts, radar recordings, while helpful, are far from definitive in accident investigations and are subject to different interpretations. Apparently with this in mind, Formica was careful to keep his declaration about a missile attack in the realm of theory.

His statement in an official forum nevertheless lent credibility to the speculation. At the same time, U.S. sources who follow terrorist activities said they have no indication that terrorists were responsible for the crash.

Formica's testimony came against a backdrop of recrimination caused by Itavia's recent financial problems, which led it to shut down flights on several routes, news agencies reported. Press accusations that Itavia planes are poorly maintained, particularly following the crash, have contributed to the airline's pinch, its officials charge.

In an apparent reference to this, Davanzali said in his letter to Formica, according to UPI: "You made us go down, and I hold you responsible. . . . My plane went down because you didn't know how to control our skies. I am certain my DC9 was hit by a missile."