Washington ? Attorney General Michael Mukasey refused Friday to give Congress details of the government’s investigation into interrogations of terror suspects that were videotaped and destroyed by the CIA. He said doing so could raise questions about whether the inquiry is vulnerable to political pressure.

In letters to leaders of the House and Senate Judiciary committees that oversee the Justice Department, Mukasey also said there is no need right now to appoint a special prosecutor to lead the investigation. The preliminary inquiry currently is being handled by the Justice Department and the CIA’s inspector general.

“I am aware of no facts at present to suggest that department attorneys cannot conduct this inquiry in an impartial manner,” Mukasey wrote to Sens. Patrick Leahy of Vermont and Arlen Specter of Pennsylvania, the top Democrat and Republican, respectively, on the Senate Judiciary Committee. “If I become aware of information that leads me to a different conclusion, I will act on it.”

Meantime, Senate Republicans blocked a bill Friday that would restrict the CIA’s interrogation methods. Already passed by the House, the bill would require the CIA to adhere to the Army’s field manual on interrogation, which bans waterboarding, mock executions and other harsh methods.

Senate opponents discovered a parliamentary flaw: The ban on harsh tactics had not been in the original intelligence bills passed by the House and Senate. Instead, it was added during negotiations between the two chambers to write a compromise bill.

That move could violate a Senate rule intended to protect legislation from last-minute amendments that neither house of Congress has had time to fully consider.

Although it’s not unheard of for new language to be added in House-Senate negotiations, the rules allow such a move to be challenged and the language stripped from the bill. Sen. Lindsay Graham, R-S.C., placed a hold on the bill while the GOP procedural challenge goes forward.

Addressing congressional demands for facts in the CIA tapes inquiry, Mukasey noted that the Justice Department generally does not give out information about pending cases.

“This policy is based in part on our interest in avoiding any perception that our law enforcement decisions are subject to political influence,” Mukasey wrote. “Accordingly, I will not at this time provide further information in response to your letter, but appreciate the committee’s interests in this matter.”

An almost identical letter was sent Thursday to Democratic leaders of the House Judiciary Committee.

Democratic and Republican lawmakers alike angrily denounced Mukasey’s refusal, which they said blocks congressional oversight of the Justice Department.