Salvage experts began last week an attempt to raise from 6,600-foot-deep ocean waters major sections of the Air India Boeing 747 that plunged into the North Atlantic June 23 and killed all 329 people on board.

A terrorist bomb is widely assumed to have caused the crash, but an international investigating team headed by an Indian judge has been unable to prove the bomb theory, and air safety specialists want to be sure. "We have 615 other Boeing 747s flying around the world," one said.

The salvage operation is the latest chapter in a high-tech mystery that involves robot submarines, extensive underwater photography and computer printouts to either confirm the bomb theory or to discover what human or mechanical failing struck the jumbo jetliner from the sky.

The evidence pointing to a bomb is circumstantial:

* A bomb is a logical explanation for the plane disappearing from an air traffic controller's radar screen without so much as a distress signal.

* Callers to U.S. newspapers claiming to represent extremist groups, the Sikh Student Federation and the Kashmir Liberation Front, claimed responsibility for the crash of the flight from Toronto to New Delhi with stops in Montreal and London.

* The two "black boxes" stopped functioning the instant the plane left the radar screen and contain little helpful information. That suggests that something cut the electrical power supply. A bomb is a possibility.

However, none of the relatively few pieces of wreckage that floated show explosion-type damage. About 130 bodies were recovered, with the deaths apparently caused by drowning or impact trauma, not obviously explosion-related.

Thus, the absence of solid evidence makes the aviation-safety community nervous, especially since the Air India crash was followed closely by the Japan Air Lines Boeing 747 disaster that claimed 520 lives.

In that crash, the aft pressure bulkhead (rear cabin wall) is suspected of having failed explosively as the plane's cabin was pressurized, damaging the tail section and controls. Boeing has said it improperly repaired the bulkhead, which was damaged in a 1978 landing incident.

The 747, with its distinctive silhouette, is the largest passenger transport ever built and the pride of Boeing's extensive handiwork. It is the plane of choice for heavily traveled, long-haul overwater routes.

The salvage ship was hired by the U.S. Navy with $700,000 provided by the National Transportation Safety Board and the Federal Aviation Administration. The Indian government has contributed about $200,000 in case more time is needed than the 15 days the $700,000 will buy.

The ship, specially fitted with a crane capable of lifting 40 tons, arrived at the crash site Tuesday carrying, among others, a specialist from the Indian investigating team and a structures expert from Boeing.

After weather delayed the start of operations, lifting began Thursday, federal officials said. Plans were to work around the clock and it was expected to take 10 to 12 hours to lift each piece. There were no progress reports from the ship yesterday, officials said. The wreckage will be taken to Ireland for analysis.

The salvage crew will try to bring up about 30 pieces of airplane that were identified and ranked in order of priority by specialists after the wreckage on the ocean floor was mapped and photographed by a robot submarine, the robot that located and retrieved the two "black boxes" from the tail section of the plane.

The sub has limited lifting ability, necessitating larger equipment. The sub's television cameras will guide salvage operators' attempts.

"We looked at 70 hours of video tape and 2,000 still photographs" to establish the priority list, one specialist said. Areas of particular interest, not surprisingly, are the walls of cargo compartments where checked luggage would have been stowed. It is assumed that if there was a bomb, it was concealed in checked baggage.

Experts also want to look at the aft pressure bulkhead to make certain there is no similarity to the damaged bulkhead found in the Japan crash. A videotape examination of the Air India bulkhead did not reveal a problem, specialists said.

"If we get real lucky, we'll get the cockpit section," one investigator said. The cockpit is thought to be the heaviest piece as well as the most awkward to handle. Cockpit instruments and switches, frozen in time, often provide valuable clues.

This is the most extensive effort to recover a plane lost in the ocean. In September 1974, a Trans World Airlines Boeing 707 departing Athens exploded. Although attempts were made to recover the craft, they were abandoned after the FBI and safety board experts, through metallurgical analysis of floating debris and expert eyewitness accounts, established that a bomb had caused the crash that killed 88 people.