A government privacy oversight board's draft report, released Tuesday night, finds the National Security Agency's use of a controversial surveillance authority is constitutional, but says some aspects of it edge up to violating the Fourth Amendment's prohibition against unreasonable searches or seizures.

The report from the Privacy and Civil Liberties Oversight Board, an independent agency within the federal government's executive branch, centers on programs revealed by former NSA contractor Edward Snowden that the NSA uses to pick up communications of foreigners that may also involve a U.S. citizen. The programs rely for legal authority on a law passed in 2008 to bring former President George W. Bush's warrantless wiretapping program under judicial oversight.

Critics charge that the NSA, in collecting the foreign targets' communications under its PRISM and upstream collection programs, incidentally picks up far too many Americans' emails and phone calls. Sen. Ron Wyden (D-Ore.) has raised alarm about what happens with Americans' communications once they're collected, because the NSA believes it can search through them without a warrant.

The privacy board's harsh January report on the NSA's separate domestic telephone metadata collection program provided ammunition for critics looking to end it. But in its new report, the privacy board largely accepts the government's assurances that NSA surveillance under the 2008 Foreign Intelligence Surveillance Act is well-grounded. The board is set to vote on whether to finalize the draft report on Wednesday.

"Overall, the Board has found that the information the program collects has been valuable and effective in protecting the nation’s security and producing useful foreign intelligence," the report reads. The program, it says, "has been subject to judicial oversight and extensive internal supervision, and the Board has found no evidence of intentional abuse."

The board acknowledges that the two NSA surveillance programs revealed by Snowden that make use of the 2008 law -- PRISM, which sends specific search terms to electronic communications service providers, and "upstream collection," in which the NSA searches the Internet backbone -- may incidentally sweep up an untold number of Americans' communications.

"The collection and examination of U.S. persons’ communications represents a privacy intrusion even in the absence of misuse for improper ends," the report finds.

Its authors express frustration that the NSA and other government agencies have been unable to furnish estimates of the incidental collection of Americans' communications, which "hampers attempts to gauge whether the program appropriately balances national security interests with the privacy of U.S. persons."

But without signs of abuse, the board concludes privacy intrusions are justified in protecting against threats to the U.S.

Nevertheless, the board suggests that the government take on the "backdoor searches" that have alarmed Wyden. In those searches, the government searches through the content of communications collected while targeting foreigners for search terms associated with U.S. citizens and residents. The House voted in June to end such searches.

The searches "push the program close to the line of constitutional reasonableness," the privacy board report says, but it doesn't recommend ending them.

As the Office of the Director of National Intelligence revealed in a Friday letter, the NSA searches communications acquired under the 2008 surveillance law for personal identifiers of Americans thousands of times per year. The CIA is also in on the act, as is the FBI, which has its own national security mission that focuses on the domestic sphere, and conducts a "substantial" but uncounted number of searches on the surveillance data.

"The manner in which the FBI is employing U.S. person queries, while subject to genuine efforts at executive branch oversight, is difficult to evaluate, as is the CIA’s use of metadata queries," the board says.

A split on the board prevented the report from recommending aggressive action on FBI searches.

Chairman David Medine and board member Patricia Wald call on a foreign intelligence surveillance court to oversee FBI searches of search terms related to Americans to determine whether they are "reasonably likely to return information relevant to an assessment or investigation of a crime."