Someone who lived with a Missouri resident infected with bird flu also became ill on the same day, the Centers for Disease Control and Prevention reported on Friday.

The disclosure raises the possibility that the virus, H5N1, spread from one person to another, experts said, in what would be the first known instance in the United States.

On Friday night, C.D.C. officials said that there was “no epidemiological evidence at this time to support person-to-person transmission of H5N1,” but that additional research was needed.

The coincidental timing of the illnesses, especially outside flu season, concerned independent experts. H5N1 has been known to spread between close contacts, including those living in the same household.

And neither the initial patient nor the household contact had any known exposure to the virus via animals or raw milk.

Neither patient has been identified, and details are scant. The household contact was not tested, so officials cannot be sure that the individual actually was infected with the bird flu virus.