(CNN) -- The National Security Agency program that collects data on nearly every U.S. phone call isn't legal, a privacy review board said Thursday in a newly released report.
Moreover, the five-member Privacy and Civil Liberties Oversight Board said it's been largely useless in thwarting terrorism.
"We have not identified a single instance involving a threat to the United States in which the program made a concrete difference in the outcome of a counterterrorism investigation," the board wrote in the report released Thursday.
The board said it had identified only one instance in which the program helped authorities identify a terrorist in the last seven years. But the board said law enforcement would have found the suspect anyway, even without the NSA program.
The board doesn't have any legal teeth, so its recommendations won't change government practices the way a court ruling might.
But the findings are a stinging rebuke of President Barack Obama's legal defense of the program, in which the NSA tracks millions of telephone calls each day, harvesting the telephone numbers involved, the time calls are placed and how long they last.
The recommendations of the board add to the growing chorus calling for an end to the government's dragnet collection of Americans' phone records, said Sen. Patrick Leahy, D-Vermont, the chairman of the Senate Judiciary Committee.
"The report reaffirms the conclusion of many that the Section 215 bulk phone records program has not been critical to our national security, is not worth the intrusion on Americans' privacy, and should be shut down immediately," Leahy said in a statement.
"The report appropriately calls into question the legality and constitutionality of the program, and underscores the need to change the law to rein in the government's overbroad interpretation of Section 215."
A majority of the board -- formed as part of the federal government's response to the September 11, 2001 terrorist attacks -- agreed with critics of the NSA that the program raises constitutional concerns.
"The Board concludes that Section 215 does not provide an adequate legal basis to support the program," it wrote its report. CNN reviewed a copy of the report.
The board said the law authorizes the FBI to collect records related to a specific investigation, but not bulk collection of data that "cannot be regarded as 'relevant' to any FBI investigation."
The law also "does not authorize the NSA to collect anything," the board said.
Last week, after a report by an outside review group appointed by Obama, the President announced changes to the program.
Among other things, the NSA must now ask a judge each time an analyst wants to look at the data, and the agency will eventually lose its role as custodian of the information.
But Obama said he felt it important to keep the program intact.
"The review group turned up no indication that this database has been intentionally abused," he said. "And I believe it is important that the capability that this program is designed to meet is preserved."
One privacy advocate called the changes "window dressing."
The government is now studying whether to require telephone companies to hold the data, or to create a new third-party agency to collect the data.