The US lawyer for Kim Dotcom says he is worried the authorities are taking advantage of an Estonian man facing charges over now-defunct website Megaupload.
Seven men, including Mr Dotcom, were originally indicted three years ago over piracy and copyright issues in the Megaupload case.
Andrus Nomm, who had previously been in New Zealand, is the first to be brought to the United States to face charges.
The Detroit News reported Mr Nomm was initially arrested in the Netherlands in 2012 and had been fighting extradition.
The newspaper said he was arrested in the US state of Virginia on Monday (local time).
Mr Dotcom's lawyer Ira Rothken said the US authorities may have taken advantage of Mr Nomm's financial situation and health condition to get him into the country.
"The last thing we knew was that the Netherlands' courts were trailing and waiting for the New Zealand courts to rule on extradition and nothing was urgent or imminent.
"But he did have a weak financial situation, he did have some healthcare issues, and we're just deeply concerned the US is taking advantage of him."
Mr Dotcom, who founded Megaupload, has been fighting extradition from New Zealand since 2012.