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“Who changed [the] talking points and deleted the references to al Qaeda?” Mr. Graham asked Tuesday, noting that officials have offered lawmakers different accounts of how and why the talking points had been altered.

A Dec. 31 report from the Senate Homeland Security and Governmental Affairs Committee noted that officials had assured Congress that no changes to the talking points were made for political reasons. But the report also said that a detailed timeline of the drafting process and all the changes, promised to the committee by Director of National Intelligence James R. Clapper, had not been delivered.

Other critics have accused the administration of treating the attack like a crime, using the FBI to try to build a legal case against the terrorists, rather than striking back with deadly force at the groups thought to be involved.

“This president is committed to ensuring that those who were responsible for the deaths of four Americans in Libya be brought to justice,” Mr. Carney said Tuesday. “There is an FBI-led investigation with that as its goal.”

The hunt for the attackers seemed to stumble Tuesday when Tunisian authorities released the only suspect who was arrested in connection with the Benghazi assault. Authorities there held Ali Harzi for months after his arrest in Turkey. Last month, he was questioned by the FBI, reportedly about an attack on the U.S. Embassy in Tunis, as well as the Benghazi assault.

Mr. Brennan, a 25-year CIA veteran who worked for Mr. Obama’s election campaign in 2008, was named Monday to replace retired Army Gen. David H. Petreaus, who resigned abruptly last year after admitting to an adulterous affair with his biographer.

Mr. Brennan’s nomination has been criticized by another Republican, Sen. John McCain of Arizona, who has promised to pressure Mr. Brennan about the agency’s use of harsh interrogation techniques against terrorism suspects during the Bush administration. Mr. Brennan oversaw the program as a senior official at the agency.

Democrats also say they have questions about Mr. Brennan’s stance on the interrogation techniques, including waterboarding.