ASSOCIATED PRESS WRITER

ROBERT BURNS

WASHINGTON (AP)—Buried deep in a once-secret Navy report is this chilling account of how the end came for a U.S. submarine and its 99 men:

‘‘The torpedo was released from the tube, became fully armed, and sought its nearest target, Scorpion.’‘

The newly declassified report says the nuclear-powered submarine USS Scorpion, which sank mysteriously in May 1968, probably went down after the torpedo’s propulsion motor started up inadvertently while in a dry tube.

The report said that while the exact cause of the accident may never be known for certain, the most likely scenario went something like this:

The crew decided to jettison the torpedo apparently because the crew had done so successfully in a similar situation a year earlier. But once in the sea, the torpedo turned on the sub.

The result was an enormous explosion that tore the Scorpion into two sections.

The torpedo was armed with a non-nuclear high explosive.

The forward hull section, including the torpedo room and most of the operations compartment, is situated in a trench on the Atlantic Ocean floor about 10,000 feet below the surface about 400 miles southwest of the Azores off the coast of Portugal.

The rear hull section, including the nuclear reactor compartment and engine room, is in a separate trench that was formed by the impact of the hull with the ocean floor.

The Scorpion is one of only two nuclear-powered submarines ever lost by the Navy. The other was the USS Thresher, which sank in 1963 off the U.S. East Coast, killing 129 men on board.

A separate Navy report released this week said that a faulty ‘‘silver-braze’‘ piping joint is one of the prime suspects in the Thresher’s sinking.

The faulty joints had nearly caused two other submarines to sink and therefore, there should have been a thorough investigation beforehand by all responsible for Thresher’s safety, the report said.

Despite that finding, the report said the loss of the Thresher was not caused by the ‘‘intent, fault, negligence or inefficiency’‘ of anyone in the Navy or at the Portsmouth Naval Shipyard, where Thresher was built and serviced just before her final voyage.

Thresher, described by one admiral as ‘‘without question the most advanced operational attack submarine in the world,’‘ was launched from the yard, which is in Kittery, Maine, in 1960. Next came tests, an extended ‘‘shakedown’‘ cruise, and nine months back at the yard for refitting and repairs.

Navy reports on the circumstances and probable causes of both accidents were prepared in the months following each accident, but were classified secret and withheld from the public.

The Navy also declassified photographs and videotape taken during 1986 expeditions to the Scorpion and Thresher accident sites by submersibles of the Woods Hole Oceanographic Institute.

The declassified documents were first reported Monday by the Chicago Tribune.

The Navy said its studies indicated that little radioactive material had leaked from the reactors on the Thresher or the Scorpion or from the Scorpion’s nuclear-armed torpedoes.

The Navy investigators concluded that the Thresher sank as a result of a piping failure.

The Scorpion investigation considered a wide variety of possible causes, including sabotage, fire, collision with a surface ship or submarine. Each was considered improbable. It also determined that there were no known Soviet or East Bloc surface warships, submarines or aircraft within 200 miles of Scorpion’s last reported position.

The report said ‘‘there is still no incontrovertible proof of the exact cause’‘ of the accident, but that the torpedo scenario was the most plausible.