MARTINEZ -- A 22-year-old Martinez mother, who allegedly offered her four-year-old child for $500 in an online conversation involving a Craigslist web personal ad posting, was released from custody Monday without being charged.

Prosecutor Dara Cashman told the San Francisco Chronicle that Shannon Nicole Woods had been released while police further investigated the incident.

Martinez police Detective Gary Peterson told reporters on Friday that his department had received a tip from the National Center for Missing and Exploited Children regarding a posting that was soliciting for prostitution.

Alleged Teen Prostitute Busted For Solicting On Craigslist

After a person who is not involved in law enforcement contacted Woods online, she allegedly agreed to meet the person for sex, said Peterson.

During the online conversation between Woods and the other person "there was an inquiry about the child and the person offered $500 for sex with the kid and (Woods) did not object to that," said Peterson.

Authorities then served a search warrant at a residence in the 200 block of Thistle Circle and placed Woods into custody on Tuesday.

Police have seized a laptop computer, several videotapes and CDs and investigators were trying to determine whether they include information indicating the child was involved in sex solicitation, according to Peterson.

Craigslist founder Craig Newmark said that the case marked the first time "this variation" of solicitation for sex had happened on the Web site, though he was not familiar with the exact posting under investigation.

When people complain about any of the thousands of postings on Craigslist, "We jump on it immediately," said Newmark. He added he is one of 18 staff members at Craigslist and that he handles two-thirds of e-mail complaints personally.

The site is primarily "self-policing" and users regularly flag postings that look like they may violate site rules.

"We're among the best on the Web at handling this kind of thing," Newmark said, explaining that Craigslist has reviewed with police across the country, the FBI and the Secret Service how to deal with criminal solicitations online. "They like how we handle things," he said.

Because Craigslist is a free online community of thousands of users, said Newmark, "We try to deal the best we can with the bad guys while respecting the whole Bill of Rights thing."