He added that prosecutors could eventually serve a company executive with the court papers.
Megaupload's founder, Kim Dotcom, is waging a legal battle in New Zealand to avoid extradition to the United States.
Prosecutors shut down megaupload.com and its related sites in January, and accused the company of criminal copyright infringement. Megaupload, which operated sites such as Megavideo.com and Megapix.com, claimed to receive 50 million daily visitors, accounting for 4 percent of total Internet traffic. According to court documents, Megavideo.com was the world's 52nd most frequently visited website.
The government said Megaupload's owners generated more than $175 million in criminal proceeds from the sites, which offered streams of movies, TV shows and other copyrighted content.