Mass collection of Americans’ phone call records by the National Security Agency is illegal and should be abandoned, according to a detailed legal and operational review of the program by an independent congressional review board.

In its first major act, the Privacy and Civil Liberties Oversight Board, set up as an independent agency in 2007 but only recently fully operational, released a 238-page report on the NSA program Thursday.

The board examined the legal foundation for the NSA metadata program that collects billions of records on the time, call duration, and phone numbers called. The practice, it found, was a clear threat to US civil liberties and of “limited value” with no significant impact on curbing terrorist plots.

The legal foundation for the program, Section 215 of the USA Patriot Act, was weak and “does not provide an adequate basis to support this program,” the board reported. Moreover, even in the dozen cases examined by the board where the metadata linked contacts to a terrorism suspect, most benefits were modest and “limited to corroborating information that was obtained independently by the FBI,” the board report found.

“Cessation of the program would eliminate the privacy and civil liberties concerns associated with bulk collection without unduly hampering the government’s efforts, while ensuring that any governmental requests for telephone calling records are tailored to the needs of specific investigations,” the report concluded.

The findings go further than the conclusions of a separate review panel appointed by President Obama, which released its report last month, or the president himself in a speech Friday. Both determined the program could be continued with thi8rd parties holding the data separately from the NSA and subject to tighter search restrictions.

But just modifying the program was not enough, the board concluded.

“The connections revealed by the extensive database of telephone records gathered under the program will necessarily include relationships established among individuals and groups for political, religious, and other expressive purposes,’’ the report said. “Compelled disclosure to the government of information revealing these associations can have a chilling effect on the exercise of First Amendment rights.”

But the board’s finding on the program's legality was a split decision. Two of the oversight board's five members dissented, saying the program could continue if further protections for civil liberties were adopted.

The two dissenters were Justice Department employees under President George W. Bush, Rachel Brand and Elisebeth Collins Cook. The three who called for a halt to NSA program include: a Federal Trade Commission official during the Clinton administration, Chairman David Medine; a retired federal appeals court judge appointed by President Carter, Patricia Wald; and a policy expert with the think tank Center for Democracy & Technology, James Dempsey.

That split seemed destined to foment further debate in Congress.

Rep. Mike Rogers (R) of Michigan, chairman of the House intelligence committee, was critical of the report.

“In 38 times over the past seven years, 17 federal judges have examined this issue and found the telephone metadata program to be legal, concluding this program complies with both the statutory text and with the US Constitution,” Bloomberg reported. “I don’t believe the Board should go outside its expertise to opine on the effectiveness of counterterrorism programs.”

But civil libertarians were heartened.

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There is “now a clear and growing consensus among the experts charged with reviewing the bulk collection program: the intrusion on Americans' privacy and the risk of abuse outweigh any minimal benefits," says Elizabeth Goitein, co-director of the Liberty and National Security Program at the Brennan Center. "First the president's own hand-picked review board, and now the executive branch's privacy watchdog committee, have concluded the program goes too far.”

A number of federal surveillance court judges as well as the Justice Department have concluded that the program is legal. Two federal judges have split on the question in recent rulings over the constitutionality of the phone data collection.