Updated, 6:15 p.m.

The head of the U.S. government's vast spying apparatus has conceded that recent surveillance efforts on at least one occasion violated the Constitutional prohibitions on unlawful search and seizure.

The admission comes in a letter from the Office of the Director of National Intelligence declassifying statements that a top U.S. Senator wished to make public in order to call attention to the government's 2008 expansion of its key surveillance law.

"On at least one occasion," the intelligence shop has approved Sen. Ron Wyden (D-Ore.) to say, the Foreign Intelligence Surveillance Court found that "minimization procedures" used by the government while it was collecting intelligence were "unreasonable under the Fourth Amendment." Minimization refers to how long the government may retain the surveillance data it collects. The Fourth Amendment to the Constitution is supposed to guarantee our rights against unreasonable searches.

Wyden does not specify how extensive this "unreasonable" surveillance was; when it occurred; or how many Americans were affected by it.

In the letter, acquired by Danger Room (.pdf), Wyden asserts a serious federal sidestep of a major section of the Foreign Intelligence Surveillance Act.

That section – known as Section 702 and passed in 2008 – sought to legalize the Bush administration's warrantless surveillance efforts. The 2008 law permitted intelligence officials to conduct surveillance on the communications of "non-U.S. persons," when at least one party on a call, text or email is "reasonably believed" to be outside of the United States. Government officials conducting such surveillance no longer have to acquire a warrant from the so-called FISA Court specifying the name of an individual under surveillance. And only a "significant purpose" of the surveillance has to be the acquisition of "foreign intelligence," a weaker standard than before 2008.

Wyden says that the government's use of the expanded surveillance authorities "has sometimes circumvented the spirit of the law" – a conclusion that the Office of the Director of National Intelligence does not endorse. The office does not challenge the statement about the FISA Court on at least one occasion finding the surveillance to conflict with the Fourth Amendment. Danger Room initially misunderstood the letter to mean that its author, top intelligence official Kathleen Turner, made the statements she was merely informing Wyden he could to issue publicly without revealing classified information.

But this is a far cry from how Director of National Intelligence James Clapper typically describes the new FISA law. When the law was up for reauthorization this spring, Clapper wrote to congressional leaders to say its renewal was his "top priority in Congress," (.pdf) as the law "allows the Intelligence Community to collect vital information about international terrorists and other important targets overseas while providing robust protection for the civil liberties and privacy of Americans."

Suspicions about abuse of the government's new surveillance powers are almost as old as the 2008 expansion of the law. In 2009, citing anonymous sources, the New York Times reported that "the N.S.A. had been engaged in 'overcollection' of domestic communications of Americans. They described the practice as significant and systemic," if unintentional. The Justice Department told the Times that it had already resolved the problem.

But as the American Civil Liberties Union noted in a May letter to lawmakers, "There is little in the public record about how the government implements" the expanded law. An ACLU Freedom of Information Act request discovered that the Justice Department and intelligence bureaucracy refer to "compliance incidents" (.pdf) in their internal accounting of the new surveillance – which seemed to suggest difficulty staying within the broadened boundaries of the law. (Full disclosure: My wife works for the ACLU.)

Wyden has been a lonely congressional voice against renewing the government's broadened surveillance powers. Last month, he quietly used a parliamentary maneuver to stall the renewal after it passed a key Senate committee.

Wyden's argument was that the government had not fully disclosed the extent of its new surveillance powers. It argued to Wyden that it is "not reasonably possible to identify the number of people located in the United States whose communications may have been reviewed under the authority of the [FISA Amendments Act]." Separately, the National Security Agency insisted that it would violate Americans' privacy even to tally up how many Americans it had spied upon under the new law.

On Friday, Wyden said in a statement: I applaud the DNI for agreeing that transparency should prevail in this situation... I believe that protections for Americans’ privacy need to be strengthened, and I believe that the FISA Court’s rulings help illustrate why this is necessary. I look forward to debating this issue on the Senate floor.”

In her letter to Wyden, Turner insisted – as the government has in the past – that all Constitutional and legal problems with the expanded surveillance have already been rectified. The government, she writes, believes the FISA Amendments Act is "a well-calibrated statute that strikes an appropriate balance between protecting national security and safeguarding privacy and civil liberties."

"At no time," she continues, "have these reviews found any intentional violations of law."