WASHINGTON — David H. Petraeus, who resigned last week as CIA director after admitting to an extramarital affair, said Friday that classified intelligence reports revealed that the deadly assault on the U.S. diplomatic mission was a terrorist attack.

But the names of suspected groups — including al-Qaida’s franchise in North Africa and a local Libyan organization, Ansar al-Shariah — were removed from the public explanation immediately afterward to avoiding alerting the militants that U.S. intelligence and law enforcement agencies were tracking them, lawmakers said.

In his first public appearance since he resigned last week, Petraeus testified before the House and Senate Intelligence Committees in back-to-back, closed-door hearings as lawmakers from both parties continued to wrestle with questions about the administration’s handling of the Sept. 11 attacks in Benghazi that killed Ambassador J. Christopher Stevens and three other Americans and why its public portrayal conflicted with the intelligence agencies’ classified assessments.

“They knew right away that there were terrorists involved in the operation,” said Rep. C.A. Dutch Ruppersberger of Maryland, the top Democrat on the House Intelligence Committee.

During his testimony, Petraeus expressed regret for his affair. Lawmakers did not ask him about it. In addition to what the administration knew about assailants, they focused their questions on possible security lapses at the diplomatic compound, particularly given a spate of attacks this year in Benghazi against the U.S. mission, the British ambassador’s convoy and the Red Cross.

State Department officials have said five diplomatic security officers were at the mission Sept. 11, including two traveling with Stevens. They were initially up against more than 50 fighters armed with automatic rifles and rocket-propelled grenades, who easily breached the compound and set fire to it.

Sen. Marco Rubio, R-Fla., said Petraeus’ testimony showed that “clearly the security measures were inadequate despite an overwhelming and growing amount of information that showed the area in Benghazi was dangerous, particularly on the night of Sept. 11.”

The talking points initially drafted by the CIA attributed the attack to terrorists. Petraeus and other top officials signed off on it, then circulated it to other intelligence agencies, as well as the State Department and National Security Council. At some point — Petraeus told lawmakers he was not sure where — objections were raised to naming the groups and the less specific word “extremists” was substituted.