ANCHORAGE, Alaska — Underwater sonar images of a black shape against a background of grainy monochrome are safely stored on two computer hard drives at Bruce Abele's home in Newton, Mass.

Blurred by odd shadows and striations, the silhouettes are the biggest clues in more than 60 years to the fate of his father's World War II submarine, the USS Grunion, which sank nearly 5,000 miles west of Massachusetts, near the obscure islands at the tip of Alaska's Aleutian chain.

For decades, relatives of the Grunion's 70 lost crewmen had no information beyond fragmented U.S. Navy records, and a few rumors, about where and why the sub went down.

They knew the Grunion had sunk two Japanese submarine chasers and heavily damaged a third in July 1942 near Kiska, one of two Aleutian islands occupied by the Japanese. They knew her last official radio message to the sub base at Dutch Harbor, on July 30, 1942, described heavy enemy activity at Kiska Harbor. They knew she still had 10 of her 24 torpedoes during that communication. They knew Dutch Harbor responded with an order to return to the base, but they don't know if Grunion ever received it.

Until a few years ago, the clues were too sparse to justify a search, said Abele, whose father, Mannert Abele, was the Grunion's commander.

"We really didn't do anything about it because there was nothing, no information," Abele said. "What were we going to do?"

Abele and his two brothers all married and had children. Bruce, the oldest, started working in computers in the late 1950s and later invested in Boston-area real estate. Brad, the middle son, owned a management recruiting business, and John helped found the multibillion dollar medical equipment company Boston Scientific Corp.

Four years ago, a man who had heard about the Grunion's disappearance e-mailed Bruce the links to several Grunion Web sites.

One site held an entirely new clue, a note from a Japanese model ship builder who said he thought he knew what had happened to the Grunion.

John Abele contacted the man, Yutaka Iwasaki, who translated and sent him a report written in the 1960s by a Japanese military officer who served in the Aleutians. A maritime magazine had recently reprinted the report.

It described a confrontation between a U.S. submarine and the officer's freighter, the Kano Maru, on July 31, 1942, about 10 miles northeast of Kiska — the Grunion's patrol area.

The sub dispatched six or seven torpedoes. All but one bounced off the boat without exploding, or missed, the officer wrote, although the hit knocked out his engines and communications. He said he returned fire with an 8-centimeter deck gun, and believed he had sunk the sub.

Japanese troops took over Kiska and Attu in early June 1942, just as the Allies were winning the battle of Midway. The U.S. Navy was shoring up its defenses in the central Pacific, but managed to assign more than a dozen submarines to the waters around Kiska at the end of the month, according to declassified Navy orders.

The Abeles began investigating the identity of the sub in the Kano Maru officer's report.

They contacted Robert Ballard, discoverer of the Titanic. He declined to participate in a search, but briefed the Abeles on the complications of searching for deep-sea wrecks. Geological formations sometimes conceal a vessel; it could be perched precariously on an undersea cliff; the water pressure and landing impact could have broken the Grunion into small pieces, making it harder to find.

They also hired a marine survey firm, Williamson and Associates, for an expedition in August to Kiska. The Seattle-based company focuses on mapping ocean and river bottoms for oil and cable companies, government agencies and academic institutions and, occasionally, explores for wrecks.

Williamson at first told the Abeles that surveying the tip of the Aleutian archipelago would be too expensive, Bruce Abele said, but after six months of negotiating, the firm agreed to send sonar technicians and equipment aboard a Bering Sea crab boat to the frigid waters licking the base of Kiska volcano.

The U.S. Navy, citing lack of resources, is not involved in the search and the Abeles prefer to keep the cost to themselves.

The Aquila, carrying more than a dozen crew members and sonar surveyors, set out from Dutch Harbor on Aug. 6, said Pete Lowney, a family friend from Newton who joined the crab fishing fleet in Dutch Harbor more than a decade ago. Lowney has fished king and snow crab for years under the Aquila's captain, Kale Garcia.

The conical volcanoes of the far western Aleutians seem to drop straight into the sea. Even in summer, rain, fog and vicious winds envelop the tiny islands.

Near the end of July 1943, for instance, the fog clung so thick around Kiska that 5,183 Japanese troops and civilians evacuated from the harbor without drawing fire from any of the surrounding U.S. battleships. The military realized a distant three weeks later that Kiska was deserted, but only after 35,000 Allied troops had spent eight days searching the fog-cloaked island, with 24 killed by friendly fire, according to the National Park Service.

For more than two weeks, the Aquila carefully towed a sonar cable from east to west and back again inside a 240-square-mile grid that the survey team had plotted using information from naval archives and the Kano Maru officer's account. The crew worked in shifts to keep the search going 24 hours a day, Lowney said.

Sonar images can deceive even those who interpret them for a living. Elongated boulders look like submarines; outcrops resemble ship's prows.

"It's a rocky seascape," said Art Wright, survey manager for Williamson. "We went over the areas several times to differentiate between rock and ship and look at things from three to four different aspects."

They looked first for the Japanese destroyer Arare, sunk by the U.S. submarine Growler, to test the sonar and see what a known wreck would look like against the seafloor. The sonar captured shapes that appeared to be two halves of the Arare, Wright said.

There were several false "eureka" moments, Lowney said.

"We put down the sonar, and I thought I saw two destroyers and got excited," he said ruefully. "After that point, I stopped jumping to conclusions."

In mid-August, the sonar picked up a 290-foot-long object with the sharp angles and jutting shadows of something man-made wedged into a terrace on the steep underwater slope of the volcano.

The Grunion, however, was 312 feet long. The Williamson team believes the bow may have plowed beneath a mat of thick sediment, hence the apparent shortage of about 20 feet. Skid marks show the vessel slid to rest about 1,000 meters from the surface, Wright said. Over the years, earthquakes along the tectonic subduction zone could have piled on more debris, he said.

Wright, a retired Navy captain who has worked with Williamson since 1986, is 95 percent sure the shadowy images are those of the vanished sub. The Grunion is the only known sunken vessel in the area, and the sonar captured the distinct outline of a submarine conning tower, he said.

"If our target is not the Grunion, where is she?" Wright said.

The Abeles remain circumspect about the find, saying they need more proof of the vessel's identity.

"Although it's very encouraging at the moment, it's dangerous to say, 'Absolutely, we have it,' " Bruce Abele said in August during a brief stop in Anchorage after the three met the crew of the Aquila on Adak, 275 miles east of Kiska.

But they have enough faith in the wreck to send out a second expedition next summer, this time with a remote-controlled underwater camera to identify the vessel and try to reconstruct her sinking.