A federal appeals judge warned the government on Tuesday that the permanent ban on speech it seeks with its FBI national security letters — which allow it to obtain records about people in terrorism investigations — was probably unconstitutional.

Judge Richard Cardamone of the 2nd U.S. Circuit Court of Appeals commented as the court acted on lawsuits challenging the government's ability to force companies to turn over information about customers or subscribers as part of the war on terrorism and keep quiet about it.

"While everyone recognizes national security concerns are implicated when the government investigates terrorism within our nation's borders, such concerns should be leavened with common sense so as not forever to trump the rights of the citizenry under the Constitution," he said.

Alluding to a recent change in federal law, a three-judge panel including Cardamone dismissed a Connecticut case as moot and returned a New York case to a lower court judge to see how the new law affects it.

Cardamone wrote a separate concurring opinion to highlight what he said was the government's recent insistence that a permanent ban on speech is sometimes permissible under the First Amendment. He said he suspected "a perpetual gag on citizen speech of the type advocated so strenuously by the government may likely be unconstitutional."

A telephone message left with a spokeswoman for the government was not immediately returned Tuesday evening.

Cardamone said courts historically have ruled in favor of the government when a ban on speech is limited so that it narrowly meets the demands of a compelling government interest. He said the bans are not constitutional once an investigation ends.

Lately, though, the government has insisted that permanent bans on speech are necessary in antiterrorism probes because all terrorism investigations are permanent and unending and grow out of other investigations, he said.

"The government's urging that an endless investigation leads logically to an endless ban on speech flies in the face of human knowledge and common sense: witnesses disappear, plans change or are completed, cases are closed, investigations terminate," he wrote.

He said a ban on speech and an unending shroud of secrecy concerning government actions "do not fit comfortably with the fundamental rights guaranteed American citizens" and could serve as a cover for official misconduct.

The comments came as the Manhattan court considered the First Amendment ramifications of challenges to government searches of Internet and telephone records, a right the government gained under a 1986 law that was expanded by the Patriot Act of 2001.

Recently, Congress made changes in the law that affect national security letters, or NSLs, investigative tools used by the FBI to compel businesses to turn over customer information without a judge's order or grand jury subpoena.

The changes specify that an NSL can be reviewed by a court and explicitly allows those who receive the letters to inform their lawyers about them.