Larimer County District Attorney Larry Abrahamson charged J.P. Weichel, 40, of Loveland, last month over posts he is accused of making on Craigslist's "Rants and Rave" section.
Colorado's libel statute, dating to the 19th century, allows criminal prosecution for speech "tending to blacken the memory of one who is dead" or to "expose the natural defects of one who is alive, and thereby to expose him to public hatred, contempt or ridicule."
Criminal libel carries a punishment of up to 18 months in prison.
"It's not a charge you see a lot of," Abrahamson said.
The case began when a woman told Loveland police in December 2007 about postings made about her between November and December 2007. Court records show one post suggested she traded sexual acts for legal services from her attorney, and there was a mention of a child services visit made because of an injury on her child.
Police obtained search warrants for records from Web sites including Craigslist before identifying Weichel as the suspect. Weichel shares a child with the woman.
Weichel, confronted by detectives at his workplace in August, allegedly said he was "just venting," according to court records. A phone number for Weichel could not be found.
In 2004, Thomas Mink filed a lawsuit challenging the constitutionality of Colorado's libel statute, and media organizations including The Associated Press and the Colorado Press Association filed "friend of the court" briefs arguing against the law.
At the time, Mink feared he would be prosecuted for libel over his Web-based journal "The Howling Pig." Greeley police had seized the former University of Northern Colorado student's computer after a professor complained about a spoof of the professor in the journal.
Prosecutors later said they did not intend to prosecute Mink. A federal judge dismissed Mink's lawsuit, but Mink filed an appeal in July in the 10th U.S. Circuit Court of Appeals.
Denver attorney Steve Zansberg, who specializes in First Amendment law, said prosecutors seeking criminal libel cases could have a "chilling" effect on free speech in Colorado, particularly over the Internet.
Abrahamson wasn't so sure. He said it's up to police departments to pursue cases.
Zansberg contends the 19th century-era law is outdated, is unclear about stating opinions, and is written in such a way that dead people could be victims of criminal libel.
Other criminal libel cases have been filed recently in Durango and in Pueblo, the Loveland Connection reported.
Weichel's libel case continues in court next month.