"It was a preliminary inquiry before, but we found the basis to open up a criminal investigation," Kristen Perezluha, a spokeswoman for the Florida Department of Law Enforcement, said Thursday. She would not elaborate.
The FBI is investigating whether Foley broke federal laws with his messages to teenaged former congressional pages, and the House Ethics Committee is looking into whether senior Republican officials hid what they knew about the messages.
Foley resigned Sept. 29 after being confronted with the lurid messages. His attorney, David Roth, has said Foley never had inappropriate sexual contact with minors. He declined to comment Thursday on the criminal investigation.
Florida law prohibits seducing or attempting to seduce a minor. However, authorities have said the term "seduce" is open to interpretation.
Foley has returned to Florida to attend his father's funeral, set for Saturday, after spending more than a month in an Arizona rehabilitation facility for alcoholism, Roth said. Edward Foley died Tuesday of cancer.
Foley entered the treatment facility Oct. 1, shortly before his attorneys announced he was gay, an alcoholic and that he had been molested by a priest as a teenage altar boy in Florida.
The Rev. Anthony Mercieca, who has retired to the island nation of Malta, has admitted having inappropriate encounters with Foley, including massaging him in the nude and skinny-dipping together. He denies having sex with Foley.
Foley has not been seen publicly since shortly after his resignation.