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Verizon had refused to comply with a subpoena the Recording Industry Association of America (RIAA) had filed in July, asking for the name of the music fan who had used the Kazaa peer-to-peer service.
However, Judge John Bates of the US District Court for the District of Columbia noted that he saw no connection between music downloading and free speech, and he ruled that the Digital Millennium Copyright Act's subpoena section is written broadly enough to allow copyright holders to include ISPs when they go digging for identities of copyright infringers.
Bates wrote that Verizon's reading of the DMCA would not allow copyright holders to identify people downloading music from peer-to-peer services. "There is little doubt that the largest opportunity for copyright theft is through peer-to-peer software, as used by the alleged infringer here.
"[Verizon's argument] would, in effect, give Internet copyright infringers shelter from the long arm of the DMCA subpoena power, and allow infringement to flourish."
Verizon vice-president and associate general counsel Sarah Deutsch said the ruling had " troubling ramifications" for computer users, service providers and the Internet, and that it opened the door for anyone who makes a copyright infringement claim to gain access to private subscriber information.
Kazaa is one of three P2P services targeted by the recording and movie industries in a lawsuit aimed at shutting down unauthorised file-sharing services .
The RIAA also issued a statement attributed to its president, Cary Sherman, which stated that the industry group looked forward to contacting the Verizon subscriber "so we can let them know that what they are doing is illegal".
In last August's motion, the RIAA said it gave Verizon a "list of literally hundreds of infringing works that were being offered for download by Verizon's subscriber and the identification of the specific location from which the alleged infringer was operating", in this case the subscriber's IP address.
Verizon lawyers argued that the DMCA's subpoena section did not address the Internet service provider - which did not have the copyrighted songs stored on any of its computers - and that it applied only to the customer who owned the computers where the songs were stored.
David Hayes, chairman of the intellectual property group of the San Francisco law firm Fenwick and West, said ISPs should be concerned about this decision, because such subpoenas now could be obtained more easily, without copyright holders having to file lawsuits. ISPs could face privacy lawsuits from subscribers if this ruling holds up, he warned.
"The copyright holders will be thrilled with this, because it gives them a powerful weapon to do investigations and learn the identities of infringers," Hayes said.