ATLANTA, Ga. - About a third of the confirmed U.S. cases of H1N1 flu are people who had been to Mexico and likely picked up the infection there, a federal health official said Saturday.

But investigations indicate many cases are getting the illness here, and that it probably still is spreading, said Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention.

In a press briefing, CDC officials said the agency knows of confirmed cases from 21 states, with Connecticut, Florida and Missouri the latest to join the list.

The CDC's count of 160 confirmed cases released Saturday is believed to already be outdated. Some states can now do their own tests for the H1N1 flu virus and don't have to send samples to the CDC. States have reported about a dozen more cases, bringing the national total to more than 170.

The H1N1 flu cases range in age from 1 to 81, but the majority are people younger than 20, said Schuchat, the CDC's interim deputy director for science and public health. Most U.S. cases have been relatively mild; only 13 have been hospitalized.

On the Net:

CDC: http://www.cdc.gov/h1n1flu