WASHINGTON — Congressional Democrats demanded yesterday that the Justice Department investigate why the CIA destroyed videotapes of the interrogation of two terrorism suspects.

The Senate’s no. 2 Democrat, Senator Durbin of Illinois, called on Attorney General Mukasey to find out “whether CIA officials who destroyed these videotapes and withheld information about their existence from official proceedings violated the law.”

Senator Kennedy, Democrat of Massachusetts, accused the CIA of a cover-up. “We haven’t seen anything like this since the 18 1/2-minute gap in the tapes of President Richard Nixon,” he said in a Senate floor speech.

And chairman of the Senate Armed Services Committee, Senator Levin, Democrat of Michigan, told reporters the CIA’s explanation that the tapes were destroyed to protect the identify of agents is “a pathetic excuse,” adding: “You’d have to burn every document at the CIA that has the identity of an agent on it under that theory.”

Democrats on the House Judiciary Committee sent letters to the director of the CIA, Michael Hayden, and Mr. Mukasey asking whether the Justice Department gave legal advice to the CIA on the destruction of the tapes, and whether it was planning an obstruction-of-justice investigation.

A White House spokeswoman, Dana Perino, said yesterday that President Bush did not recall being told about the tapes or their destruction. But she could not rule out White House involvement in the decision to destroy the tapes, saying she had only asked the president about it, not others.

Ms. Perino refused to say whether the destruction could have been an obstruction of justice or a threat to cases against terrorism suspects. If the attorney general decides to investigate, “of course the White House would support that,” she said.

In a daily press briefing dedicated almost solely to the topic of the CIA tapes, Ms. Perino responded 19 times that she didn’t know or couldn’t comment.

The spy agency destroyed the tapes in November 2005, at a time when human rights groups and lawyers for detainees were clamoring for information about the agency’s secret detention and interrogation program, and Congress and American courts were debating where “enhanced interrogation” crossed the line into torture.

Also at that time, the Senate Intelligence Committee was asking whether the videotapes showed CIA interrogators were complying with interrogation guidelines. The CIA refused twice in 2005 to provide the committee with its general counsel’s report on the tapes, according to the chairman of the committee, Senator Rockefeller, Democrat of West Virginia.

Mr. Hayden told agency employees Thursday that the recordings were destroyed out of fear the tapes would leak and reveal the identities of interrogators. He said the sessions were videotaped to provide an added layer of legal protection for interrogators using new, harsh methods. President Bush had just authorized those methods as a way to break down the defenses of recalcitrant prisoners.

Destruction of the tapes came in the midst of an intense national debate about how forcefully prisoners could be grilled to get them to talk. Not long after the tapes were destroyed, Congress adopted the Detainee Treatment Act, championed by Senator McCain of Arizona, who was tortured while a prisoner of war in Vietnam. The law prohibits not only torture, but cruel, inhuman, and degrading treatment of all American detainees, including those in CIA custody.

Also in the fall of 2005, the Supreme Court heard a case involving the legal rights of detainees held at Guantanamo Bay Naval Base. It decided in June 2006 that Al Qaeda prisoners are protected by the Geneva Conventions’ prohibitions on torture and cruel treatment.

At the time, the CIA also was concerned that its operatives involved in prisoner interrogation might be subject to legal charges over the treatment of detainees. Some agency employees have bought liability insurance as a hedge against that possibility.

The decision to destroy the tapes was made by Jose Rodriguez, then the head of the CIA’s clandestine directorate of operations under the director of the CIA, Porter Goss.

Mr. Hayden said members of the congressional intelligence committees were made aware in February 2003 both of the tapes and the CIA’s ultimate plan to destroy them. That claim was denied by several members of the panels, including Rep. Peter Hoekstra of Michigan, who at the time was chairman of the House Intelligence Committee.

The Senate Intelligence Committee did not learn of the tapes’ destruction until November 2006. The House Intelligence Committee learned of the tapes’ destruction in March 2007.

Republicans were mostly mum about the CIA disclosure. Mr. McCain, a presidential candidate, said while campaigning in New Hampshire yesterday that would not side with Democrats’ calls for an investigation because he believed the CIA’s actions were legal.

“That doesn’t mean I like it,” Mr. McCain added.

“Of course I object to it,” he said of the tapes being destroyed. “Right now, our intelligence agencies need credibility and this is not helpful to that.”

At least one of the tapes showed the interrogations of Abu Zubaydah, the first high-value detainee taken by the CIA in 2002. Zubaydah, under harsh questioning, told CIA interrogators about an alleged 9/11 accomplice, Ramzi Binalshibh, Mr. Bush said in 2006. The two men’s confessions also led to the capture of Khalid Sheik Muhammed, whom the American government said was the mastermind behind the September 11, 2001 terrorist attacks.

Mr. Hayden told agency employees the interrogations were legal, and said the tapes were not relevant to “any internal, legislative, or judicial inquiries.”

Lawyers for American detainees said they believe otherwise.

The Center for Constitutional Rights, which coordinates the work of all attorneys representing American prisoners held at Guantanamo Bay Naval Base, says the CIA may have destroyed crucial evidence a court said it was entitled to in 2004.

The center said yesterday it is now “deeply concerned” the CIA may have destroyed evidence relating to a former CIA detainee now held at Guantanamo, Majid Khan.

Revelations about the tapes also may affect ongoing terrorism trials.

Lawyers for a convicted terrorism conspirator, Jose Padilla, claimed in a Florida federal court that Zubaydah was tortured into saying Padilla was an Al Qaeda associate. The Justice Department dismissed Padilla’s allegations as “meritless,” saying Padilla’s legal team could not prove that Zubaydah had been tortured.

Padilla and his two co-defendants will be sentenced next month. They face life in prison on three terror-related convictions.

A former U.S. District Judge, Mr. Mukasey, now attorney general, signed the warrant used by the FBI to arrest Padilla in May 2002. That warrant relied in part on information obtained from Zubaydah, court records show.

In a separate case, attorneys for Al Qaeda conspirator Zacarias Moussaoui in 2003 began seeking videotapes of interrogations they believed might help their client. In November 2005 a federal judge ordered the government to disclose whether it had video or audiotapes of specific interrogations. Eleven days later, the government denied it had them.

The tapes also were not provided to the 9/11 Commission, which relied heavily on intelligence reports about Zubaydah and Binalshibh’s 2002 interrogations. A CIA spokesman, Mark Mansfield, said the agency did not subvert the 9/11 commission’s work.

“Because it was thought the commission could ask about tapes at some point,” he said, “they were not destroyed while the commission was active.”