The co-founder of a group advocating for an Army private accused of leaking classified material to the antisecrecy website WikiLeaks is suing the U.S. government for unlawfully seizing his computer and copying its contents to aid a criminal investigation of the site.

Computer scientist David House’s laptop was taken in November at an international airport by two Department of Homeland Security agents without a hint that it contained evidence of wrongdoing, but rather because House was a vocal supporter of Pfc. Bradley Manning, the accused leaker, the American Civil Liberties Union alleged in a complaint to be filed today in U.S. District Court in Boston.

The case, the civil liberties advocates contend, is a troubling instance of how the government’s more aggressive border search policies in the post-Sept. 11 era are being used not to enforce customs or immigration laws, but to advance government investigations of third parties and to collect information about people’s political activities.

The seizure of House’s laptop was unconstitutional, they argue, because it contained such a vast amount of personal material – including private membership lists – that reviewing it would be akin to probing House’s thoughts.