Washington ? Atty. Gen. John Ashcroft has decided to claim the sweeping power to keep undocumented immigrants in custody indefinitely for national security reasons even if immigration judges rule to release them on bond.

Ashcroft’s decision came in the form of an opinion he signed last Friday that only became widely known this week when immigration lawyers and others obtained copies of the document. It was requested by the Homeland Security Department, which now has authority over most immigration matters.

Because the attorney general cited the requirements of national security for his decision, some critics and civil liberties advocates quickly accused him of taking advantage of post-Sept. 11 fears to advance a conservative agenda. They also charged him with wasting federal resources to corral people who aren’t threats to the United States.

“Detention is one of the most severe powers that a government has,” said Stephen Yale-Loehr, an immigration-law authority who teaches at Cornell Law School in Ithaca, N.Y. “To be able to detain someone on general considerations of national security raises serious issues for all Americans.”

Detainees are often held for months — some more than a year.

While the opinion dealt specifically with the case of an 18-year-old Haitian immigrant who had sought asylum, experts expected the opinion would serve as a precedent that could be applied to undocumented immigrants of almost any nationality.