Scientists say poisoning from heavy metals could be partly responsible for the deaths of thousands of fish last year in the Potomac River and its tributaries.

The fish kills began in May, just south of Washington, when 35,000 fish washed up dead on Virginia and Maryland beaches. Several thousand more died in Potomac Creek in July. Other deaths have occurred along the length of the Potomac below Stafford, Va.

Scientists are unsure whether the deaths were caused by natural conditions in the water, pollutants seeping into the river, fertilizers washed from farm fields by rain or illegally dumped toxins.

"We really don't know, and we may never know," said Cynthia Sale, water resources manager for the northern regional office of Virginia's State Water Control Board.

But tests on fish from the first kill on the Potomac River near Fort Belvoir, conducted by the Maryland Department of Natural Resources, showed kidney damage that suggests the fish could have been poisoned by metals such as lead.

No single industry or polluter has been found.

Local wardens with the Virginia Department of Game and Inland Fisheries also have followed the investigations of the kills as observers.

"We don't really see a common thread," said biologist John Odenkirk. But over the years, there have been enough rumors and sightings of dead fish on the Potomac Creek to raise concerns that illegal dumping may be one of the problems.

Summer can bring lower water and oxygen levels in creeks and rivers. That can kill fish or stress them, making them more susceptible to disease or poisons, Odenkirk said.

Summer thunderstorms can quickly drop several inches of rain, sending pollution from parking areas or farm fields into already low streams, leaving a potent dose of poisons.

All of those factors could have played a role, Odenkirk said.