PORTLAND – The deaths of 21 large whales found floating in New England waters in July might have been caused by poison in microscopic algae, federal researchers said Wednesday.

Domoic acid, a neurotoxin that occurs naturally in the ocean, was found in samples taken from some of the dead humpbacks and other whales.

Those preliminary findings were disclosed by researchers working with the National Marine Fisheries Service at a meeting of the U.S. Marine Mammal Commission in Newport, R.I.

More tests are being conducted to confirm the suspicion that the whales died after eating plankton or fish containing the poison. The dead whales were found about 200 miles southeast of Portland.

Although domoic acid has killed sea lions and other animals off the West Coast of the United States, it has never been linked to the deaths of whales or other marine mammals on the East Coast.

Domoic acid first was identified by scientists in 1987 when several Canadians died and more than 100 were sickened from eating Prince Edward Island mussels that had filtered the toxin out of the water. Some of those who survived suffered permanent memory loss.

“It attacks the brain,” said Mark Wells, an associate professor at the University of Maine’s School of Marine Sciences. “It causes nerve cells to fire continuously and die.”

Domoic acid has never been found in dangerous levels along the Maine coast where it could taint clams or mussels, said Amy Fitzpatrick of the Maine Department of Marine Resources.

Federal and state officials cautioned that the findings do not necessarily mean that domoic acid was a factor in the deaths of at least nine whales and dozens of seals found along the Maine coast in September and early October. Samples from some of those animals are being tested for domoic acid as well as other potential culprits.

Both clusters of marine mammal deaths were highly unusual, but they may well be unrelated, said Teri Frady, a spokeswoman for the National Marine Fisheries Service’s Northeast Science Center.

“I would resist linking this particular event to the [dead animals found] in Maine,” she said.

Reports of harmful or toxic algal blooms have increased around the world in recent years. Last year, for example, almost 100 whales and dolphins beached in Southern California, apparently because of domoic acid poisoning.

Some have linked the algal blooms to such factors as global warming, increased pollution and ship traffic that carries organisms around the globe.

David Townsend, director of the University of Maine’s School of Marine Sciences, said the more likely reason for the increasing reports is that scientists only recently have begun studying the toxins.

“The more you look, the more you see,” he said.