The US Justice Department is trying to block a bid by Kim Dotcom to take a formal role in a court action brought by a Megaupload client who wants his seized files back.
A number of Megaupload users can no longer access their files since the site was shut in January last year by order of a court in Maryland.
Claimant Kyle Goodwin says he used the site legitimately and wants a court to give him back files needed for his business.
Megaupload, says it can provide important information in the case and has asked to be a formal party in the court proceedings.
But in papers filed in court in the United States, the Justice Department says Megaupload wants to take part only so it can find out more about the government's case against it.