WASHINGTON — Sometimes cyberstalking is necessary, law enforcement officials and legal experts said Monday — especially if the emails in question reflect an inside knowledge of the CIA director.

Some commentators have questioned whether the bureau would ordinarily investigate a citizen complaint about unwanted emails, suggesting that there must have been a hidden motive, possibly political, to take action. FBI officials are scheduled to brief the Senate and House intelligence committees Tuesday about the case.

But law enforcement officials insisted Monday that the case was handled “on the merits.” The cyber squad at the FBI’s Tampa, Fla., field office opened an investigation, after consulting with federal prosecutors, based on what appeared to be a legitimate complaint about email harassment.

The complaint was more intriguing, the officials acknowledged, because the author of the emails, which criticized Jill Kelley for supposed flirtatious behavior toward David Petraeus at social events, seemed to have an insider’s knowledge of the CIA director’s activities. The emails suggested that their author knew Petraeus and had witnessed his interactions with Kelley.

“There was a legitimate case to open on the facts, with the support of the prosecutors,” said one official, speaking on condition of anonymity because the case remains open.

He added, “They asked, does somebody know more about Petraeus than you’d expect?”

Kelley, a volunteer with wounded veterans and military families, brought her complaint to a rank-and-file agent she knew from a previous encounter with the FBI office. But the officials said the agent was “just a conduit” for the complaint and did not get it special attention.

Law enforcement officials said that as the FBI investigation progressed, it raised a series of security issues: whether sexually explicit emails to Broadwell were in fact from Petraeus, or from someone who had hacked his account; and where she had obtained classified documents found on her computer. But the questions were resolved, and he was determined to have no criminal liability.

A government official clarified Monday that FBI agents’ first interview with Paula Broadwell — at which she is said to have admitted having had an affair with Petraeus, and voluntarily allowed agents to search her computer — took place in September. An earlier account had put that interview during the week of Oct. 21.

Before Broadwell spoke to the FBI agents, Petraeus had learned that she had sent offensive emails to Kelley and asked her to stop, another official said. By the time agents interviewed the CIA director during the week of Oct. 28, he was aware of the cyberstalking investigation and readily acknowledged his affair with Broadwell, the official said.