The co-founder of a group advocating for an Army private accused of leaking classified material to the antisecrecy Web site 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 Friday 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. They say that the government should have a suspicion of a crime and a “border-related” justification to conduct such searches.

A Justice Department spokesperson declined to comment. But a former senior DHS policy official said the plaintiff has a difficult case because the courts have traditionally placed few limits on the government’s border search authority.

Ordinarily, a search warrant based on probable cause of a crime would be needed to examine a person’s laptop, but when a traveler is entering the country, the government’s position is that it does not need a warrant — or even reasonable suspicion. In this case, House was stopped at Chicago’s O’Hare International Airport on a return trip from Mexico.

As a general rule, the U.S. Supreme Court has held that as long as a search is routine or reasonable, the intent of the search does not matter. But the Supreme Court has not ruled on the specific issue of whether searching a laptop with troves of personal data is reasonable without at least some suspicion of a crime.

The government held House’s laptop for 49 days. On it were several years’ worth of e-mails with family, friends and co-workers; passwords to his bank account and workplace computer; confidential messages of the Bradley Manning Support Network about strategy and fund-raising; and lists of potential donors and notes on donor meetings.

“The computer is like an extension of my mind,” House said in an interview. “It is my notes, my writing, locations I’ve been.”

What concerns him, he said, is that the government appeared to be targeting him because of his advocacy work and may now start targeting others he associates with. “All these people working for the Manning Support Network, all of a sudden their names are in the open, and that is most worrisome,” he said.

Manning, who has been detained by the military since last May, faces 22 criminal charges for allegedly leaking material. Separately, a federal grand jury in Northern Virginia has been exploring possible criminal charges related to WikiLeaks’ publishing of sensitive material, including diplomatic cables and reports on the wars in Afghanistan and Iraq. The Manning Support Network, House said, is not affiliated with WikiLeaks.

House, who lives in Cambridge, Mass., said he thinks the government seized the laptop in a “fishing expedition” to further its investigation of WikiLeaks and its founder, Julian Assange.

News of the seizure last fall caused potential donors to back away — a chilling, House said, of his First Amendment right of association. The publicity also led to calls last year for his dismissal as a researcher at the Massachusetts Institute of Technology, he said. (House eventually left to become a freelancer.)

David Cole, a law professor at George Washington University, said that if the government seized House’s laptop not for “valid customs law enforcement purposes, but rather to get intelligence about protected political associations or as a way of avoiding the requirements of criminal law searches, then it seems to me they’re abusing the border search exception for purposes it was never designed to serve.”

Paul Rosenzweig, a former senior DHS policy official, noted that the Supreme Court has ruled that “searches made at the border . . . are reasonable simply by virtue of the fact that they occur at the border.”

He said that the only court-imposed limit has been for body-cavity searches, which require reasonable suspicion.

“It’s absolutely clear that if you were carrying around your Rolodex in your briefcase across the border, the government can take it, photocopy it and give it back to you without probable cause or a reasonable suspicion,” he said.

The George W. Bush and Obama administrations have taken the position that searching a laptop is no different than examining a suitcase, and that such a search requires no suspicion. An appeals court in California has upheld that view in a case involving a traveler suspected of having child pornography on his laptop.

The appeals court erred, the ACLU contends. Moreover, the group said, House’s computer contained membership lists, not child porn.

Since last September, House has reentered the countr y seven times, and each time he was stopped and questioned. This has led him to think that his name is on a watchlist — not because he is suspected of terrorism or any crime, but because of his advocacy work.

In three encounters, including when his laptop was taken, he was asked about the Manning support group and whether he was affiliated with WikiLeaks. He was asked no questions relating to terrorism or customs laws, and at no point did agents suggest that he had broken the law or that his computer contained illegal material, he said.