WASHINGTON — The collapse of the impressive career of CIA Director David Petraeus was triggered when a woman he was having an affair with sent threatening e-mails to another woman close to him, according to three senior law enforcement officials with knowledge of the episode.
The recipient of the e-mails was so frightened that she went to the FBI for protection and help tracking down the sender, according to the officials. The FBI investigation traced the threats to Paula Broadwell, a former military officer and a Petraeus biographer, and uncovered explicit e-mails between Broadwell and Petraeus, the officials said.
When Petraeus’s name surfaced, FBI investigators were concerned that the CIA director’s personal e-mail account had been hacked and that national security had been threatened. The officials said further investigation, including interviews with Broadwell and Petraeus, led to the discovery that the two were having an affair.
The identity of the woman who received the e-mails was not disclosed, and the nature of her relationship with Petraeus is unknown. The officials said the woman did not work at the CIA and was not Petraeus’s wife, Holly.
The law enforcement officials said the e-mails indicated that Broadwell perceived the other woman as a threat to her relationship with Petraeus.
Attempts to reach Broadwell and her relatives have been unsuccessful, and she has not made a public statement since she was linked with Petraeus on Friday. All three senior officials who described the impetus for the investigation spoke on the condition of anonymity because aspects of the inquiry are ongoing.
Petraeus, a retired four-star Army general who was once seen as a potential presidential candidate, said Friday that he was resigning as CIA chief because he had been involved in an extramarital affair. He has been married for almost 40 years and has two grown children. Broadwell is married and has two young children.
In an e-mail sent to a longtime friend Friday night, Petraeus expressed regret for letting down his family and the nation. The friend, who described the contents of the message on the condition of anonymity, said Petraeus conveyed profound remorse in the message.
‘‘He was deeply sorry for the pain he has caused his family,’’ the friend said. ‘‘He also noted how much he loved his job at the agency. He said he really relished the intellectual challenge there.’’
Other details emerged Saturday indicating that the Petraeus allegations became a secret election-night drama for the Obama administration. That evening, the Justice Department informed the director of national intelligence, James Clapper Jr., that their investigation had unearthed compromising information about the CIA director, according to a senior US intelligence official.
Clapper then spoke with Petraeus and urged him to resign, notifying the White House the next day. That sequence has become a source of controversy, raising questions among some members of Congress about why key intelligence committees were not notified earlier and why the FBI waited before informing the administration about an inquiry that had stumbled onto embarrassing details about the CIA chief.
The law enforcement officials did not provide an exact timetable for the investigation, but they said the inquiry started several months ago. They said investigators thought they were dealing with a routine harassment case until some communications were traced to a private e-mail account belonging to Petraeus.
The sexually explicit nature of the communications caused investigators to suspect that someone had broken into Petraeus’s e-mail account, leading to concerns about potential national security breaches, according to the officials.
As the investigation proceeded and more evidence emerged, including Broadwell’s role, FBI investigators realized they had uncovered an affair between Petraeus and Broadwell, the officials said.
The e-mails from Broadwell indicated that she thought the other woman was becoming involved with Petraeus, according to the officials. They said the e-mails were ‘‘threatening and harassing’’ but not specific enough to warrant criminal charges.
One of the officials said that the recipient of the e-mails complained to Petraeus about them and that the FBI later obtained e-mails between Petraeus and Broadwell in which they discussed the harassment.
The investigators first interviewed Petraeus about two weeks ago, the officials said. They reviewed the evidence with him but did not suggest that he should resign or that he would be charged with a crime, according to the officials.
One of the officials said Justice Department officials were unclear on what steps to take after they concluded that there would be no charges and that there had been no breach of national security.
The notification finally came Tuesday evening, while polls were still open across the country. The official said Clapper has not called for his office or CIA to conduct a follow-up investigation or damage assessment, indicating that Clapper does not see the case as a security threat.
Still, the timing of Petraeus’s departure, and the apparent decision by the FBI to withhold information about its inquiry, is already coming under question and criticism from Capitol Hill.
Senior Senate aides said the Senate intelligence committee did not learn of the matter until Friday, just hours before the Petraeus resignation was announced. Even then, the first word came from news reports, prompting the committee to press the White House and CIA for answers.