ANNAPOLIS, Md. – Police have arrested four women in an investigation into prostitution ads that were posted on a popular Internet classifieds site.

The women were arrested over the last three weeks at hotels near Baltimore-Washington International Thurgood Marshall Airport, police said. They were charged with illegally soliciting sex.

Anne Arundel County authorities began the investigation after reading ads posted on Craigslist.org that offered sex services for $140 to $300.

Police said Tuesday the women allegedly arranged several weeks’ worth of appointments in advance, then flew into a city to meet with clients. They then traveled to their next destination.

Experts said Craigslist has become a favorite way for prostitutes to find clients and set up operations in cities where they don’t have a network. Classified ads in a variety of categories are free on Craigslist.

“The Internet facilitates the mobility and the outreach to clients, and there are a number of Web sites, including Craigslist, that these workers use to announce their presence in a particular city during a particular day,” said Ronald Weitzer, a sociology professor at George Washington University.

Police said two of the four women are from California and Illinois and the others live in Washington, D.C. and Laurel. Trials are pending.