More than 40 state attorneys general are set to warn major peer-to-peer file-sharing networks that they may face enforcement actions if they do not take steps to stem illegal activity on the networks, such as the trading of child pornography and stolen movies and music.

In a letter to the heads of Kazaa, Grokster, BearShare, Blubster, eDonkey2000, LimeWire and Streamcast Networks, the attorneys general write that peer-to-peer (P2P) software "has too many times been hijacked by those who use it for illegal purposes to which the vast majority of our consumers do not wish to be exposed."

The letter, which could be sent as early as today and was obtained yesterday by The Washington Post, is the first time state law enforcement officials have thrown their combined weight against the P2P networks, which allow free sharing of digital files -- movies, music, software, photos and so forth -- among millions of computer users.

The letter does not threaten immediate or specific action against the networks, but it does say, "We will, as appropriate, continue to initiate such actions in the future to stop deceptive and illegal practices by users of the Internet, including users of P2P software" if the networks do not take "concrete and meaningful steps" to prevent illegal use of their networks.

Courts have ruled that the networks are not liable for acts of their users, but those cases are under appeal. Further, the attorneys general who signed the letter say the ruling does not fully exempt the networks from enforcement actions.

The P2P networks do not have a central server and thus are hard to police. Files reside on the hard drives of computer users and can be accessed via high-speed Internet connections even when the computer is shut off. More than 40 million Americans use the networks; Kazaa is the most popular. Movie and music companies blame P2P-enabled illegal file-sharing for lost revenue.

The entertainment industry has fought the networks, attempting to shut them down. When the courts rebuffed this tactic, the music industry began suing individual users of the networks.

The P2P groups are fighting a bill introduced by Sen. Orin G. Hatch (R-Utah) that would allow copyright holders to sue the makers of products that are deemed to "induce" copyright violations.

"This letter is full of falsehoods, exaggerations, things that have been taken care of or are about to be completely addressed, so we think this letter is old news," said Marty Lafferty, chief executive of Distributed Computing Industry Association, a trade group representing Kazaa. "But we take our relationship with the state AGs very seriously and we will respond to them and try to give them a better understanding of P2P technology and where the industry is going."

The attorneys general recently asked the P2P networks to address the illegal use of their networks. But their answers "failed to address the issues raised by P2P software," the letter says.

The state officials also ask the networks to stop adding encryption features to their networks that they say prevent law enforcement agencies from policing the networks to determine whether they are aiding illegal activity.

The encryption measures "only reinforce the perception, as well as the reality, that P2P technology is being primarily used for illegal ends," the letter says.