Adding a new challenge to their list of legal attacks, the record industry and Hollywood studios have joined forces to sue Music City, Kazaa and Grokster, which together form one of the most popular file-trading networks to spring up in Napster's wake. The suit marks the fourth major legal action the copyright holders have filed in their attempt to restrain millions of people from trading copies of songs and movies online. So far they have been successful, shutting down Scour, stopping trades on Napster and putting smaller outfit Aimster on the financial ropes. But the new lawsuit marks the first overseas defendants, as well as the first legal test of a new generation of technology, which may prove a harder beast to pen than have the previous file-swapping services. Unlike Napster, the file-swapping services sued on Tuesday don't require a central company to create an index of downloadable files. The companies involved have said that even if they disappear, the network of file swapping can survive as people continue to distribute the software unofficially. That's not stopping the Recording Industry Association of America (RIAA) and the Motion Picture Association of America (MPAA) from targetting the network, dubbing it a "21st century piratical bazaar." "We cannot sit idly by while these services continue to operate illegally, especially at a time when new legitimate services are being launched," said RIAA chief executive Hilary Rosen in a statement. Perhaps more than any previous legal action, the latest lawsuit will test the ability of courts and one nation's system of law to reach across international borders for an online issue, and over technological hurdles that many techno-libertarians have deemed all but impassable. The source of the software for all three services is a company alternately called FastTrack, or Consumer Empowerment, based in Amsterdam. A self-described "virtual organisation" of programmers from Sweden, Denmark and the Netherlands, the company has licensed its file-swapping software to Music City -- incorporated in Oregon, but headquartered in Tennessee -- and the West Indies-based Grokster. FastTrack chief executive Niklas Zennström also manages Kazaa. While each company gives its own brand to the service, the software underlying each is virtually identical, and a file-trader using any one of them taps into the same loose network of computers. Music City's Morpheus and the Kazaa Media Desktop have been among the most popular downloads on the Internet in recent months, with more than 34 million versions of the program downloaded from CNET's Download.com, one of the most popular download sites on the Web. Grokster has trailed the other, nearing 200,000 downloads. The software reports how many computers can be accessed at any given time. The number of people reported by the network has held steady at about 600,000 people online simultaneously in recent weeks. Collectively these and other rivals have kept file swapping alive and well in Napster's absence. Consulting firm Webnoize estimated that 3.05 billion files were downloaded using the FastTrack-based network, Audio Galaxy, iMesh and the Gnutella network during August. That compared with a similar estimate of 2.79 billion files downloaded through Napster in February 2001, the peak of that service's popularity. The FastTrack software is a second-generation peer-to-peer service, which some analysts have said will be far harder to control than Napster or its other early rivals. Napster, Scour and others like them allowed people to swap files by connecting into central servers that created a constantly updated index of files available for download on people's hard drives. To download a file, a direct link would be established between two computers. No files actually flowed through the companies' central servers, but without them no index could be created. The FastTrack-based network, like the open-source Gnutella technology, instead allows searches to ripple through individual computers in the network without ever going through a centralized company server. Each company maintains Web pages, advertising streams and bulletin boards that show up through the software, but all of these bits of information are irrelevant to the process of searching and file swapping. The upshot of all this, peer-to-peer boosters have said, was that the companies themselves could be sued out of existence, but the file-swapping network could still remain. The new lawsuit will finally put that theory to the test. The RIAA and Music City, at least, already have a history of discussions. The record companies contacted the company earlier this year after winning an appeals court victory over Napster, and notified executives that they were running a file-swapping service that was likely in violation of the law. Music City was using different software at that time, and shortly afterward switched to the FastTrack-based Morpheus Music City executives could not immediately be reached for comment. In previous discussions, chief executive Mike Weiss mentioned plans for a more comprehensive piece of software that folded the file-swapping service into others that might include communications services. In recent days, Music City has begun touting itself as an underground media distribution outlet. "Many important pictorial as well as audio and video news events are never allowed to reach television or print media," the company says on its Web site, which is displayed inside the application. "With Morpheus, you can create and share the real news as you see it around the world." The move appeared one way to try to establish that the software has "substantial non-infringing uses," a legal term of art that can protect whole technologies from being ruled illegal. The VCR, for example, was protected by the courts in a suit by movie studios because it can be used for many things other than making copies of copyrighted movies. In the case against Napster, judges have been so far been unconvinced that non-infringing uses, such as promoting unsigned bands, were enough to allow unrestricted file-trading on Napster's network to go on without check. That decision has not yet come to a full trial, however. Napster has settled part of its ongoing lawsuit, agreeing to pay music publishers $26m in return for that group dropping its copyright infringement claims. Record companies continue to pursue Napster in court, however. The FastTrack-based system will present a slightly different calculus for courts if it goes that far, since music, software and video can be traded. This was the case for Scour as well, but that company went bankrupt before its case came to court. The case was filed late Tuesday in a federal court in Los Angeles. See the MP3 News Section for the latest headlines. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Napster Debate. Let the editors know what you think in the Mailroom. And read other letters.