FREDERICKSBURG, VA. -- Scientists are trying to determine what is causing thousands of fish to turn up dead in two Stafford County creeks.

The fish, mostly shad and carp, have been found dead in Aquia and Potomac creeks recently, and a larger kill at Potomac Creek surfaced over the weekend. Officials said the fish, which have been dead for several days, were stirred from the bottom of the creek by a thunderstorm.

A similar fish kill occurred in the Potomac River two months ago, when about 30,000 fish turned up dead.

Fish within a two-mile section of Potomac Creek have large, open lesions and other signs of illness.

Albert Giles, a supervisor at the State Water Control Board, said all types of fish had been affected.

"When that affects all varieties, this tends to lower the expectation that it's a disease," said Giles, who is heading an investigation of the latest kill. "Some of the lesions are extremely deep, right down to the bone."

Investigators caught some live fish to send to Richmond for testing. Investigators haven't ruled out toxic pollution, but they say the fish kills could be related to a natural fungus.

John Odenkirk, a biologist with the Department of Game and Inland Fisheries, said low dissolved oxygen levels in the creeks might play a part in the recent deaths. Odenkirk said insufficient oxygen in the water could be stressful to the fish, which lose the ability to fight off diseases.

He blamed the hot weather for reducing dissolved oxygen in the water. Sewage and other materials dumped into the river could also compound the problem.