The RIAA learned of the swappers' identities after awith Verizon Communications, which unsuccessfully fought attempts to unmask its subscribers, citing concerns about privacy and legal liability.
Four of the five persons whom the letters targeted are the Verizon subscribers involved in the legal case. An RIAA representative said on Thursday that the fifth recipient is a subscriber of Internet service provider EarthLink, which agreed to turn over the individual's name after an appeals court panel ordered Verizon to unveil the identities of its piracy-suspected subscribers earlier this month.
The RIAA would not identify the suspects' names, nor would it comment on whether the subscribers had responded to the letters or whether it planned to follow up with further legal action.
The record labels have stepped up their pursuit of individual file swappers in recent months.
In April, the RIAA filed its first lawsuit against students whom it suspected of peer-to-peer piracy. Until then, it had sued companies that provided file-sharing technologies, not the people who used them. In May, the four studentsthat required each student to pay the RIAA between $12,000 and $17,000.
A recent court decision also may bolster the RIAA's plan to pursue individuals rather than companies. The RIAA had argued that the parent companies of the Grokster and Morpheus services violate copyright law by providing software that enables piracy. In April, a judge disagreed, saying the makers of Grokster and Morpheusfor copyright infringement and thus leaving the record labels little alternative but to go after individuals who use the services.
Following the Grokster ruling, RIAA CEO Hilary Rosen warned that "individual infringers cannot expect to remain anonymous when they engage in this illegal activity."