Scientists also say the tests prove it takes only a few of the invasive species, as few as just a breeding pair, to cause the type of population explosions seen in a Crofton pond in 2002 and in portions of the Potomac last year.
"It doesn't take many individuals to start a problem," said Thomas Orrell, a biologist at the Smithsonian Institution's Museum of Natural History. "If you dump a male and female in there, it is going to be a big problem if they breed."
Orrell tested snakeheads found in Crofton, Wheaton, the Potomac, Philadelphia and Shrewsbury, Mass.
DNA tests from 16 mostly juvenile snakeheads from the Potomac found all but one were related, meaning a small number of the invasive species produced all those found in the river.
The fish that did not match was an adult male that could have parented the brood since the test traces only DNA passed on by a female.
Maryland and Virginia scientists fear that the fish, native to China, has established a breeding population -- threatening native species such as largemouth bass.
About 20 snakeheads were caught last year in the Potomac, many of them young, although no direct evidence was found to prove they were reproducing.
Anglers caught two more last week in Virginia, said Steve Early, a biologist with the Maryland Department of Natural Resources.
While DNA testing shows most of the Potomac fish are related, there is the outside possibility that all were bred elsewhere and dumped into the river at the same time, he said.
"We are still a ways away from having a well-established population that is going to endure," he said.
Northern snakeheads are aggressive predators, and scientists fear they can dominate an ecosystem if introduced. They are a delicacy in China and are sold in some Asian markets domestically.
Some private collectors also keep them in tanks. They can live out of water for short periods and can wriggle between waterways.
But biologists believe the fish are sometimes dumped into waterways when they outgrow their tanks or are no longer wanted.
In the Crofton case, a man admitted to dropping two into the small pond after buying them in a New York market. More than 1,000 were found, and the DNR eventually poisoned the water to kill them all.
That year, a snakehead was discovered in a Shrewsbury pond. Last year, a fish was found in a Wheaton pond, and several were caught in a Philadelphia lake.
Anglers caught several on the Virginia and Maryland sides of the Potomac River, leading fisheries officials to search for signs the fish were reproducing. The states plan further sampling this year.
The Smithsonian DNA tests showed that the Crofton fish were related but did not match those found elsewhere, allaying fears that the pond was the source of the other snakehead discoveries.
"It means we did in fact eradicate everything at Crofton pond," Early said. "The source for the Potomac River is somewhere else."