PORTLAND (AP) – Scientists investigating high mortality rates of seals say no fast answer has emerged.

Reports of stranded seals have slowed to normal rates in recent weeks, but researchers continue to search for clues about what happened to the harbor seals last summer in Maine.

“We’re peeling the onion, but we’re still finding more layers,” said Greg Early, a Massachusetts researcher and one of the leaders of the federal team investigating Maine’s seal deaths.

The mystery began in July with a surge of calls to the Northeast Marine Animal Lifeline. The Westbrook-based network rescues sick or injured seals, tests and examines them and nurses the animals back to health. The calls continued at an unprecedented pace through August, says Greg Jakush, founder of the lifeline.

“We’re at case 766 right now for the year,” Jakush said. “Our average is 350 to 375 per year.”

Most of the animals have been harbor seals, which are common to the Maine coast. The reports included 305 dead harbor seal pups and 164 dead adult seals. Those numbers are up dramatically from past years.

The lifeline’s rehabilitation center bustled with as many as 47 sick or injured seals at one time this summer, straining its finances and the network of volunteer rescuers who rush to beaches when the hotline receives reports.

The phenomenon made headlines in late August after the discovery of 27 dead adult harbor seals on Stratton Island in Saco Bay.

“That was about 30 animals, over a two- to three-week period, out on a little haul-out,” Early said. “And that was it. After that it stopped. That was unusual by anybody’s measuring stick.”

Scientists say it’s possible the Stratton Island die-off and the other reports are separate incidents. But the island also is in the middle of a well-defined stranding zone that stretches from Kittery to Boothbay.

“That spot is like the glowing red center of things,” Early said.

As a result of the reports, the National Marine Fisheries Service declared an “unusual mortality event,” and assigned a team scientists to investigate.

Scientists in labs in South Carolina, Washington, D.C., Connecticut and Cape Cod have been testing tissue and blood samples from the Maine seals, as well as examining entire carcasses found on beaches here.

Commercial fisheries, endangered whales and humans all rely on healthy oceans, Early says, and scientists see dying seals as a potential warning.

“It’s the canary in the coal mine,” he said.