WASHINGTON — The Justice Department has reopened a long-dormant inquiry into the government’s warrantless wiretapping program, a major policy shift only days into the tenure of Attorney General Mukasey.

The investigation by the department’s Office of Professional Responsibility was shut down last year, after the investigators were denied security clearances. A former attorney general, Alberto Gonzales, told Congress that President Bush, not he, denied the clearances.

“We recently received the necessary security clearances and are now able to proceed with our investigation,” counsel for the OPR, H. Marshall Jarrett, wrote to Rep. Maurice Hinchey, Democrat of New York.

Mr. Hinchey and other Democrats have long sought an investigation into the spying program to see if it complies with the law. Efforts to investigate the program have been rebuffed by the Bush administration.

“I am happily surprised,” Mr. Hinchey said. “It now seems because we have a new attorney general the situation has changed. Maybe this attorney general understands that his obligation is not to be the private counsel to the president but the chief law enforcement officer for the entire country.”

The OPR investigation was begun in February 2006 but was shut down a few months later when the National Security Agency refused to grant Justice Department lawyers the security clearances to ask questions about the program. Justice Department officials said Mr. Gonzales recommended Mr. Bush approve the clearances, but the president said no.

White House officials referred questions to the Justice Department.

The investigation “will focus on whether the DOJ attorneys who were involved complied with their ethical obligations of providing competent legal advice to their client and of adhering to their duty of candor to the court. Because this matter involves a pending inquiry, I can’t comment further,” a Justice Department spokesman, Brian Roehrkasse, said in a statement.

The Office of Professional Responsibility was created to ensure that Justice Department lawyers do not violate ethical rules. It is not authorized to investigate activities of the National Security Agency.

Mr. Bush’s decision to authorize the spy agency to monitor people inside America, without warrants, generated a host of questions about the program’s legal justification.

The administration has vehemently defended the eavesdropping, saying the NSA’s activities were narrowly targeted to intercept international calls and e-mails of Americans and others inside America with suspected ties to the Al Qaeda terror network.

A separate Justice Department internal investigation was opened last year by the agency’s inspector general. Those investigators received their security clearances around the same time the OPR investigators’ were denied, and their probe is ongoing.

Democrats have complained in the past that neither probe reviews whether the surveillance program violates the Constitution, the kind of decision usually reserved for the courts.