Washington ? President Bush effectively blocked a Justice Department investigation of the National Security Agency’s warrantless surveillance program, refusing to give security clearances to attorneys who were attempting to conduct the probe, Atty. Gen. Alberto Gonzales said Tuesday.

Bush’s decision represents an unusually direct and unprecedented White House intervention into an investigation by the Office of Professional Responsibility, the internal affairs office at Justice, according to administration officials and legal experts. It forced OPR to abandon its investigation of the role played by Justice Department officials in authorizing and monitoring the controversial NSA eavesdropping effort, according to officials and government documents.

“Since its creation some 31 years ago, OPR has conducted many highly sensitive investigations involving Executive Branch programs and has obtained access to information classified at the highest levels,” the office’s chief lawyer, H. Marshall Jarrett, wrote in a memorandum released Tuesday. “In all those years, OPR has never been prevented from initiating or pursuing an investigation.”

In testimony Tuesday to the Senate Judiciary Committee, Gonzales said that in matters involving access to classified programs, “the president of the United States makes the decision.”

“The president decided that protecting the secrecy and security of the program requires that a strict limit be placed on the number of persons granted access to information about the program for non-operational reasons,” Gonzales wrote in a related letter sent to the committee’s chairman, Sen. Arlen Specter, R-Pa. “Every additional security clearance that is granted for the (program) increases the risk that national security might be compromised.”

The program, begun in secret after the Sept. 11, 2001, attacks and revealed in news reports in December, allows the NSA to intercept phone calls and e-mails between the United States and locations overseas without court approval if one of the parties is suspected of links to terrorist groups. It is the focus of several lawsuits and months of wrangling between the administration and Congress over its legality.