HARTFORD, Conn. -- Forty states have reached an agreement with the Web site Craigslist to crack down on ads for prostitution, Connecticut Attorney General Richard Blumenthal said Thursday.

"This identifying information, like telephone numbers and credit card information, can provide a roadmap," said Blumenthal. "We will use this roadmap to track down and lock up prostitutes, sex traffickers, anyone engaging in this illegal activity."

Under the deal, anyone who posts an "erotic services" ad on Craigslist will be required to provide a working phone number and pay a fee with a valid credit card. The Web site will provide that information to law enforcement if subpoenaed to do so.

"We have fairly tight controls on the kind of telephone numbers that can be used for this," said Jim Buckmaster, president of Craigslist. "Of course there's going to be some illegitimate phone numbers that slip through, but the beauty of this is we see an inappropriate ad in addition to suspending the account, we now blacklist that phone number and the person has got to come up with a different phone number."

Craigslist has also agreed to sue 14 software and Internet companies that help people who post erotic service ads to circumvent the Web site's defenses against inappropriate content and illegal activity.

Craigslist will also begin using new search technology in an effort to help authorities find missing children and victims of human trafficking.

Police across the country have been arresting people for using Web sites like Craigslist to advertise the sexual services of women and children.

"The dark side of the Internet must be stopped from eclipsing its immense potential for good," Blumenthal said in a statement Thursday. "We'll continue to fight illicit and illegal activity on the Internet, especially threatening the safety and well-being of children."

The agreement between the states and Craigslist was joined by the National Center for Missing and Exploited Children.

"The criminals engaged in the sexual trafficking of children no longer parade them on the streets of America's cities," said NCMEC President and Chief Executive Ernie Allen. "Today, they market them via the Internet."

States that signed the agreement include Arizona, Arkansas, Colorado, Connecticut, Delaware, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Mississippi, Montana, Nebraska, Nevada, New Hampshire, New Mexico, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin and Wyoming.

Washington, D.C., the U.S. Virgin Islands and Guam also joined the agreement.