"It's not a charge you see a lot of," Larimer County District Attorney Larry Abrahamson said of the 1800s-era state law that can put people in jail for the content of their speech or writing.
Abrahamson charged J.P. Weichel, 40, of Loveland, in October over posts he allegedly made on Craigslist's "Rants and Rave" section.
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 posts that suggested she traded sexual acts for legal services from her attorney and mentioned a visit from child services because of an injury to 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, said he was "just venting," according to court records.
No phone listing could be found for Weichel, and his attorney, Michael Liggett of Fort Collins, didn't immediately return a message left Monday by The Associated Press.
Libel is commonly seen as a civil case. 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 is up to police departments to pursue cases.
Zansberg contends the law is outdated, is unclear about stating opinions and is written in such a way that dead people could be victims of criminal libel.