For a moment early this week, Virginia scientists thought they might have finally found ground zero for the northern snakehead's invasion of the Potomac River.

On Sunday, the 19th snakehead of the summer was caught in Fairfax County's Mulligan Pond, near the far upstream reaches of snakehead-rich Dogue Creek.

It was a good spot for snakeheads: shallow, muddy, full of underwater grass. And during hard rains, the pond overflows into the creek, so snakeheads from the pond would have had an outlet to the Potomac.

"It just seemed so natural, so logical" that the original snakehead could have been dumped there, said John Odenkirk, a fisheries biologist at the Virginia Department of Game and Inland Fisheries,

But when scientists combed the pond using electroshocking equipment and large nets, they came up empty. There were hundreds of other fish -- gizzard shad, carp, crappie -- but no other snakeheads.

Once again, it seemed that the Summer of the Snakehead would end with more questions than answers about the alien invader.

The northern snakehead, which first appeared in the Washington region in a Crofton pond two years ago, made headlines again when it suddenly appeared in the Potomac this spring. Also this summer, the same species of fish popped up in Philadelphia and eastern Massachusetts.

"We thought the summer of 2002 was the big year for snakeheads," said Walter R. Courtenay Jr. of the U.S. Geological Survey. "That pales in comparison to this."

The northern snakehead is a native of China and Korea that can grow to several feet long. It has large teeth, a voracious appetite for other fish and the ability to wriggle short distances over land.

It was a troubling sign when the fish, usually imported for food or aquariums, appeared in the open water of the Potomac. Biologists feared it would gobble up native fish or out-compete the river's famous stock of largemouth bass.

But scientists said they still don't know whether the fish is breeding in the river.

Certainly, there is strong circumstantial evidence of reproduction: The snakeheads caught in the Potomac have been of several different ages, and some were females full of eggs.

Even more convincing, Odenkirk said, fishermen have reported seeing pairs of snakeheads performing their equivalent of a mating dance: swimming in circles in a bed of underwater grass, the male and female nipping at each other's tails.

"It certainly seems that they're trying to spawn," Odenkirk said.

But so far, no scientist has found baby snakeheads, which would be the definitive proof.

The results of a DNA analysis at the Smithsonian Institution, which could determine whether the snakeheads have a common mother, are expected back this month.

Scientists are also at a loss to explain why the snakeheads -- which had been lunging at fishermen's lures all summer -- suddenly stopped biting in August.

Sixteen fish were caught from May to mid-July, but only three have been caught since then. Two snakehead fishing tournaments came up empty.

A similar phenomenon has puzzled scientists in Pennsylvania, where northern snakeheads were found this summer in a Philadelphia lake. Six fish were caught there in July, but only one since, said a spokesman for the Pennsylvania Fish and Boat Commission.

Some wonder if the drop-off is related to a decline in fishing: Maybe anglers stayed off the water on hot August days. The most optimistic scientists wonder if it means that most of the Potomac's snakeheads have been caught.

"I hope it's good news," said Steve Early of Maryland's Department of Natural Resources.

Another possibility: This might be a time of year when snakeheads stop feeding or move away from their summer homes in shallow creeks.

Scientists can't say, simply because they know so little about how the fish behaves here.

The search for the snakehead's point of origin also goes on. Odenkirk said the fish probably was dumped in one of three Fairfax streams near Mount Vernon: Little Hunting Creek, Dogue Creek or the confluence of Accotink and Pohick creeks.

That search, though, probably will end temporarily with this summer's fishing season, Odenkirk said.

"I suspect that next spring we'll be back," he said.

Steve Owens nets a large carp as he and fellow Virginia Department of Game and Inland Fisheries worker Scott Herrmann pulled electrically stunned fish from Pohick Bay on the Potomac River in May while searching for northern snakeheads. Nineteen snakeheads have turned up in the Washington area this year.