Attorneys general from 45 states sent letters to seven companies that offer online file-sharing software, hinting at possible legal consequences if the networks don't better inform computer users about potential copyright violations from sharing files.
But a legal expert questioned how file sharing might break state laws.
The move signals the states' willingness to go after the purveyors of Kazaa, Morpheus and other similar peer-to-peer software, which entertainment companies contend are profiting from the unauthorized distribution of songs, movies and software by users of their programs.
The letter was signed by attorneys general from all but five states -- Alaska, Kansas, Nebraska, New Hampshire and Wyoming -- the District of Columbia and the U.S. Virgin Islands.
The attorneys general ask the companies to improve how they inform those who use their software about potential legal and security risks associated with file-sharing, which include being sued for copyright infringement, identity theft, and unwittingly being exposed to pornography, computer viruses and spyware.
The companies are also urged to develop better filters for pornography, but not to make any changes to their software, such as adding encryption features to hide users' identity.
"Encryption only reinforces the perception that P2P technology is being used primarily for illegal ends," the letter says. "Accordingly, we would ask you to refrain from making design changes to your software that prevent law enforcement in our states from investigating and enforcing the law."
The letter stops short of spelling out consequences if the companies don't heed the requests, but it includes references to past legal action taken by the states against suspected spammers.
It's unclear what legal action based on state law is open to the attorneys general.
Efforts to use federal copyright laws to shutter the current crop of file-sharing software distributors have stalled since the original file-sharing network, Napster, was forced to close down in 2001.
But states can only enforce copyright violations when it applies to sound recordings made before 1972, said Fred von Lohmann, senior intellectual property attorney at the Electronic Frontier Foundation in San Francisco.
"I'm not aware of any state law that file-sharing violates," von Lohmann said. "This letter is clearly an exercise of political clout on the part of the entertainment industry."
Letters were sent to the companies behind Kazaa, Morpheus, Grokster, Bear Share, Blubster, MetaMachine/EDonkey 2000 and Lime Wire, as well as two of the so-called P2P industry's trade associations.
Adam Eisgrau, executive director of P2P United, a trade group that represents several of the firms, questioned the jurisdiction of the attorneys general.
In a statement, Lime Wire LLC in New York said it would provide additional warnings to its users "as appropriate," but rejected the suggestion to add filters or remove encryption.