Attorney General Eric H. Holder Jr. on Thursday defended the FBI’s handling of an investigation into a sex scandal that forced the resignation of CIA Director David H. Petraeus, saying investigators found no national security threat that would have warranted alerting President Obama and senior lawmakers.

“We do not share outside the Justice Department, outside the FBI, the facts of ongoing investigations,” Holder told a news conference in New Orleans as he addressed the issue publicly for the first time. “We made the determination as we were going through the matter that there was not a threat to national security. Had we made the determination that a threat to national security existed, we would of course have made that known to the president and also to the appropriate members on the Hill.”

Holder said Justice Department officials “felt very secure in the knowledge that a national security threat did not exist that warranted the sharing” while the investigation was underway. He said a turning point came late in the investigation, after investigators conducted a final interview on Nov. 2 with Paula Broadwell, the Army Reserve officer who wrote a biography of Petraeus and became his mistress.

“When we got to that point where we thought it was appropriate to share the information, we did so,” Holder said. By then, the FBI had already interviewed Petraeus, who acknowledged the affair.

The FBI cleared Petraeus of mishandling classified information, but Broadwell was found to have classified material in her possession and is still under investigation, news agencies reported. Her security clearance was revoked.

Holder said the probe was conducted “in the way that we normally conduct criminal investigations ... in an impartial way.” He made the comments at a news conference called to announce that oil giant BP has agreed to plead guilty to 14 criminal counts and pay $4 billion to settle a case stemming from a 2010 oil spill in the Gulf of Mexico.

Separately, the CIA said Thursday that its inspector general is opening an “exploratory” investigation into the conduct of Petraeus, who headed the agency from September 2011 until his resignation Nov. 9.

Top CIA and FBI officials came under questioning about the scandal Thursday during congressional hearings originally called to probe an attack that killed the U.S. ambassador to Libya and three other Americans in Benghazi, Libya, on Sept. 11. Petraeus is scheduled to testify before Congress on Friday about the Benghazi attack.

The affair came to light after a Tampa socialite complained to an FBI agent about anonymous hostile e-mails that warned her to stay away from Petraeus, law enforcement sources have said.

The Tampa FBI agent never sought to blow a whistle on the case, according to a person close to the agent, but instead made informal comments that were eventually relayed to a top Republican lawmaker.

The account contradicts descriptions of Fred Humphries, identified Wednesday as the agent involved, as a whistleblower who was trying to expose Petraeus’s extramarital affair with his biographer and its possible national security risks. It also suggests that, save for informal comments by an FBI agent not working on the case, Petraeus’s affair may not have come to light as quickly as it did.

Humphries knew little about the status of the FBI’s investigation, the person close to him said, and had only remarked about the intriguing focus of the case to a friend. That friend, who has not been identified, contacted Rep. Dave Reichert (R-Wash.), who then arranged for Humphries to speak with House Majority Leader Eric Cantor (R-Va.).

The Tampa FBI agent has been described as having had concerns about the case and characterized as a whistleblower in numerous press accounts.

“He’s not a whistleblower,” the person close to Humphries said in an interview Thursday.

Humphries’ attorney, Lawrence Berger, echoed those comments, saying his client has engaged in no improper conduct.

Members of Cantor’s staff have said that Humphries raised concerns about the case, saying he was worried national security had potentially been breached.

Cantor’s staff members have said the Virginia lawmaker talked to Humphries on Oct. 27 and then contacted a prominent lawyer in his state, former U.S. attorney Richard Cullen, for advice. Delayed by the government shutdown associated with Hurricane Sandy, his chief of staff contacted Mueller’s chief of staff on Oct. 31.

At the time, the case was moving forward. Petraeus had been interviewed by FBI agents on Oct. 29. Mueller’s office assured Cantor’s staff that the case was being fully investigated.

One week later, the Justice Department disclosed the existence of the investigation to James R. Clapper Jr., the director of national intelligence, who informed the White House. Petraeus resigned three days later.

Among the questions being asked by congressional leaders is why the White House and Clapper, who was Petraeus’s supervisor in the intelligence community, were not notified earlier. Some have suggested that Cantor’s contact with the FBI led to the disclosure.

“I don’t know if it would have taken this course without Cantor,” a person close to the inquiry said.

Humphries is a veteran FBI agent who had previously been a handler for a key al-Qaeda operative.

Law enforcement officials said that, while his actions are being reviewed in the Petraeus case, he is not under formal investigation.

Media reports have indicated that Humphries sent a shirtless photo to Jill Kelley, the friend who first contacted him about harassing e-mails, later traced back to Broadwell, the woman with whom Petraeus was having an affair.

A person close to Kelley said the photograph was a joke — a photo of Humphries without a shirt next to two dummy torsos on a firing range. A law enforcement official said the photo was sent at least six months before the investigation into the harassing e-mails began.

Karen DeYoung contributed to this report.