WASHINGTON  The Justice Department finalized on Friday an overhaul of rules that will give the Federal Bureau of Investigation freer rein to begin investigations into the possibility of terrorism, even without evidence of wrongdoing.

Attorney General Michael B. Mukasey approved the new guidelines after a review of several months that sought to consolidate different and sometimes conflicting standards within the F.B.I. that govern when agents can use informants, do undercover surveillance, interview witnesses or use other investigative techniques.

Mr. Mukasey and Robert S. Mueller III, director of the F.B.I., said the new guidelines, which will take effect Dec. 1, “provide the F.B.I. with the authority and flexibility it needs to protect the nation from terrorist threats.”

The new guidelines, reflecting the evolution of the F.B.I. in the seven years since the Sept. 11 attacks, state that “the F.B.I. is an intelligence agency as well as a law enforcement agency.” They are also one of the final steps by the Bush administration to extend its far-reaching counterterrorism policies into the next administration and beyond.