MIKE STOBBE - AP Medical Writer

NEW YORK (AP) -- Genetic analysis of recent monkeypox cases suggests there are two distinct strains in the United States.

That raises the possibility that the virus has been circulating undetected for some time.

Federal health officials described the genetic analysis Friday. Many of the U.S. cases were caused by the same strain as recent cases in Europe.

But a few samples show a different strain.

Each strain had been seen in U.S. cases last year, before the recent international outbreak was identified.

Health officials say analysis from many more patients will be needed to determine what's going on.