Matthew Muller, the prime suspect in a California kidnapping that’s become known as the “Gone Girl” case because police initially believed the victim faked the crime, agreed to tell a reporter a few things “off the record” in a jailhouse interview. One of those things was apparently that he did it, Wired’s Kevin Poulsen reports.

Although KPIX-TV reporter Juliette Goodrich wasn’t allowed to record the interview or even write it down, Muller’s apparent confession made it into the record anyway—the jail records conversations between inmates and visitors, and forwarded the tape of the interview to the FBI.

Muller is accused of kidnapping a woman named Denise Huskins last March after drugging Huskins and her boyfriend and demanding their bank account numbers, Wi-Fi password, and other internet account passwords.

When Huskins reemerged two days later, hundreds of miles away, police at first suspected she had faked the kidnapping to frame her boyfriend, Aaron Quinn—a setup similar to the plot of Gillian Flynn’s revenge novel Gone Girl.

The FBI determined that wasn’t the case—instead, they suspected Muller, a Harvard Law grad and former Marine who had been arrested in a separate home invasion.

In a 9,000-word manifesto mailed to the San Francisco Chronicle, the kidnappers claimed to be a group of three “gentleman thieves” who were trying to pull a few big jobs and use the ransom to retire. The writer claimed the group had let Huskins go after determining they’d kidnapped the wrong woman.

A tape the suspect played for Quinn and Huskins while they were bound with zip-ties in their apparent also described “a professional group collecting on financial debts,” the FBI said.

But in the taped jailhouse interview, Wired reports, Muller claimed there was no such group. Wired reports:

[A]ccording to the FBI affidavit, the gang was just one of Muller’s fantasies.

“In the context of discussing the kidnapping of [Huskins] ‘off the record and on background,’ Muller said that there was no gang and that it was just him,” special agent Wesley Drone wrote in the affidavit.

The interviewer, Goodrich, also said that Muller described his declining mental health over the past half-decade.

The FBI now believes Muller had carried out, or attempted, a number of similar home invasions around Silicon Valley as early as 2009, and that he’d done extensive research on the victims beforehand.

“The fact that Muller knew facts about his victims indicates that he probably conducted online research of his targets,” Drone wrote in the affidavit, per Wired. “These recurring crimes involving nighttime home intrusions in which passwords are demanded indicates some plan for further use of computers and/or the Internet for crime.”

The two alleged 2009 burglaries, committed when Muller was still a practicing lawyer, each had the same M.O.: a woman drugged with Nyquil and interrogated about her computer password. Muller was stopped and questioned about one of them in the middle of the night, but was never charged due to lack of evidence.

Wired also notes that two months after being questioned by police about those crimes, Muller disappeared, leaving his wife a note that he was going “off the grid.”

“I have problems beyond my mental health,” he wrote, “I live in terror most of the time and can’t keep up appearances any longer, and this is perhaps the least extreme thing I can do to resolve it that does not also expose everybody to criminal liability.”

He’s also suspected—but hasn’t been charged—in another 2012 break-in and attempted rape.

Muller has pleaded not guilty to the suspected home invasion that resulted in his arrest last June.

[Photo provided by Muller’s attorney to ABC News]