Scientists have discovered the possible presence of bird flu in the United States — in wild swans near the banks of Lake Erie — but it does not appear to be the worrisome strain that the government has long feared.

Routine tests on two seemingly healthy wild mute swans in Michigan suggest they might have the H5N1 virus; confirmatory tests are under way.

But other testing has ruled out that it could the so-called highly pathogenic version of that virus that has ravaged poultry in Asia, and killed at least 138 people worldwide, the Agriculture Department announced Monday.

"This is not the highly pathogenic avian influence virus that has spread through much of other parts of the world," said Ron DeHaven, administrator of USDA's Animal and Plant Health Inspection Service.

"We do not believe this virus represents a risk to human health," he declared.

Still, if it is the low-pathogenic version of H5N1, it wouldn't be the first time even that version of H5N1 has been spotted in the United States. The low-pathogenic virus was found in wild ducks in 1975 and 1986, and on a Michigan turkey farm in 2002, USDA officials said. A similar low-pathogenic version was found in Canada last year.

"They believe it is a strain of low pathogenicity, similar to strains that have been seen before in North America," White House spokesman Tony Snow said.

Despite the lack of human risk, federal officials said they were announcing the apparent discovery as a sign of openness about the $29 million effort to check for signs that H5N1 is posing a renewed threat to U.S. birds.

"We remain vigilant and prepared," said Dr. William Raub, science adviser to the Department of Health and Human Services.

The deadly version of the H5N1 virus has killed at least 138 people worldwide since beginning its global march in late 2003. But virtually all caught the virus from close contact with sick birds or their droppings.

Scientists had feared that the deadly form of the virus would reach North America — in birds — sometime this year. Just last week, the U.S. expanded monitoring of wild migratory birds throughout the nation, to check for early signs.

Health officials are closely watching H5N1 for fear the virus eventually could mutate into a strain that could spread easily from person-to-person, possibly sparking a worldwide epidemic. No one knows how likely that is to happen, and specialists agree that the risk doesn't jump even if a few infected birds are found to have entered the U.S.

Still, even the low-pathogenic H5N1 requires monitoring, because it has the potential to mutate into the highly pathogenic form — the kind that rapidly kills birds, especially poultry. If it were found in the U.S., that would trigger additional security steps to prevent wild birds from infecting commercial poultry flocks.