A word of advice to jail inmates who give press interviews: "Off the record" doesn’t mean squat to the FBI agents listening in.

That's one lesson accused kidnapper Matthew D. Muller is learning the hard way. Muller, 38, has been in custody since June at the Santa Rita Jail in Alameda County, California, facing state charges for a home invasion and federal charges for the high-tech kidnapping of a California woman. Today Muller's attorney is asking a judge to throw out the physical evidence seized from his client—items like a pair of swim goggles converted to a blindfold, five aerial drones, and a Super Soaker mocked up to look like a laser-sighted handgun—arguing that the police were led to the evidence by an illegal search of Muller's cell phone.

If that argument succeeds, though, the federal government has a secret weapon: a bugged conversation between Muller and a local TV reporter, in which Muller confessed to the kidnapping, "off the record and on background."

It's the latest twist in a case that began last March, when 29-year-old Denise Huskins was taken from her Vallejo, California, home, held for 48 hours, then released 400 miles away in Southern California with no ransom paid. She later said she had been sexually assaulted during the abduction. The kidnapping was strange in a number of ways, including its high-tech theatrics. The perpetrator drugged Huskins and her boyfriend, interrogated them for personal information and online passwords, attempted to monitor the aftermath of the crime with a webcam, and used anonymous remailers, image sharing sites, and Tor to communicate with the police and the media during and after the crime.

The Vallejo police announced at the time that the whole thing had to be a hoax staged by Huskins and her boyfriend. But then a second, abortive home invasion occurred in nearby Dublin last June. This time the victims fought off the attacker, and a cell phone abandoned at the scene led law enforcement to Muller, a Harvard Law School graduate and former immigration attorney with a history of mental health issues.

Muller has pleaded not guilty in the Dublin case, and faces federal kidnapping charges for the Vallejo kidnapping. But a new FBI affidavit filed last month—but not reported until now—reveals that Muller came close to being arrested six years ago, in 2009, for two very similar crimes against women in Silicon Valley, in an investigation that prompted him to briefly leave his wife and go into hiding. In the end, no charges were filed.

The FBI document also shows that the government has an audio recording of Muller confessing to the Vallejo kidnapping—potentially a key piece of evidence in the case.

The Jail Has Ears

That recording was made in July, when news of Muller's arrest first broke, and local KPIX-TV news reporter Juliette Goodrich enterprisingly signed up to see Muller during the Santa Rita jail's normal visiting hours.

Visitations at Santa Rita are tightly controlled, with the visitor and inmate separated by a window and speaking over a phone. Goodrich wasn't allowed to bring a camera crew or even paper and pen with which to take notes. Instead she hurriedly wrote down her recollections from the interview as soon as it was over.

In the resulting broadcast, Goodrich said that Muller "didn’t deny" a role in the Vallejo kidnapping and that he described six years of worsening mental health. She also noted that she couldn't report everything that Muller said, because he "made clear that certain things would be off the record and for background only."

Now, we know what those "certain things" were, because the jail recorded Muller's conversation with Goodrich, and gave a copy of the recording to the FBI.

According to the FBI’s summary—not necessarily an unbiased account—Muller confessed to the reporter and cleared up one of the mysteries hanging over the case: Did he act alone, or does he have accomplices still at large?

This is a June 2015 booking photo released by the Dublin, Calif., police department, showing Matthew Muller after he was arrested on robbery and assault charges. On Monday, July 13, 2015, Muller was named as a suspect in the kidnapping and sexual assault of a woman from Vallejo in March of 2015 that police originally believed was a hoax. (AP Photo/Dublin PD) Dublin PD/AP

Huskins told the police that she thought there were multiple kidnappers holding her. And the anonymous emails later sent to the police and press buttressed that theory, describing a three-person gang that began as an auto-theft ring before deciding that kidnapping-for-ransom would be more lucrative.

But according to the FBI affidavit, the gang was just one of Muller's fantasies.

Acting Alone

"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 FBI filed the document in support of a search warrant to scour various electronic devices seized from Muller, including several laptops and cellphones taken from a house in South Lake Tahoe where Muller is believed to have held Huskins, and a hard drive recovered from his car, which the FBI found last month stashed in long-term parking at the Reno-Tahoe International Airport. The FBI expects to find evidence that Muller cyber-stalked his victims before each home-invasion, and perhaps used their online accounts afterward.

"The fact that Muller knew facts about his victims indicates that he probably conducted online research of his targets," Drone wrote. "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."

Muller's attorney didn't return phone calls for this story.

A String of Alleged Crimes

The FBI affidavit also adds new detail about two 2009 home-invasions in Palo Alto in which Muller was a suspect.

In the first one, on September 29, a masked early-morning intruder broke into a Mountain View, California, apartment, where he handcuffed and bound the 27-year-old woman who lived there. Claiming he was there to commit identity theft, he demanded the woman's date-of-birth, mother's maiden name, and Social Security number and asked her questions about her computer and DSL line—behavior identical to the Vallejo kidnapping last March. He also placed blacked-out swim goggles over the victim's eyes as a blindfold and sedated her with an overdose of Nyquil, just as in the Vallejo case.

Then on October 18, a nearly identical home invasion—complete with Nyquil—occurred at the Palo Alto apartment of a 32-year-old woman. In both cases the intruder threatened to rape the victims.

Detectives investigating the Palo Alto case in 2009 reviewed police reports of prowlers in the area and discovered that four days before the first attack an attorney named Matthew D. Muller had been stopped in Palo Alto in the middle of the night after a resident reported a suspicious man seen exiting one apartment building and walking into another

At the time, Muller was still a respected immigration lawyer, and he talked his way out of the stop by claiming he was a visiting scholar from Harvard University teaching at nearby Stanford.

The detectives discovered that the Palo Alto victim had a connection to Muller: She'd attended a policy panel organized by Muller at Harvard the year before. When the cops contacted Muller, he agreed to be interviewed but then lawyered up instead. Two months later, he vanished, leaving a note for his then-wife on a USB drive. "I'm going completely off the grid, no phone, email, credit cards, etc., so please do not try to track me as it will only draw attention," he wrote. "Any commotion at the apartment is also not likely to go unnoticed."

"I have problems beyond my mental health," he wrote his wife in a second message. "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."

Muller phoned his wife from Utah two days later, and she picked him up. Despite the strange behavior and the circumstantial evidence against him, the Palo Alto police believed they did not have enough evidence against Muller, and no charges were filed.

The FBI also suspects Muller in another November 2012 case in Palo Alto, in which a masked intruder broke into a young woman's home and attempted to rape her. After a struggle, the attacker ran off, leaving behind a set of "bump keys"—a key blank cut in such a way that, with some practice, it can be used to open common residential-grade locks in a few seconds.

Illegal Search?

Muller is scheduled to appear in court today to challenge the evidence against him in the Dublin case. At issue is the cell phone he allegedly left at the scene. When police first arrived, they found the phone locked, so they immediately called 9-1-1 from the phone and got the number from the dispatcher. Muller's defense attorney argues that the call constituted an illegal search, because the police had no warrant.

This undated photo released by the Vallejo Police Department shows Denise Huskins. Police say Huskins, who was reported kidnapped from her boyfriend's San Francisco Bay area home and held for ransom, has contacted her father to say she's in the Southern California city of Huntington Beach. (AP Photo/Vallejo Police Department) Vallejo Police Department/AP

"I think it's a good motion," says Jennifer Granick, director of civil liberties at the Stanford Center for Internet and Society. "The officers manipulated his phone to send data to the dispatcher so they could collect it without a warrant." But the fact that the phone was abandoned may weigh against Muller. "I think the easiest way for a judge to rule against the defendant will be because he left his cell phone at the scene."

The cell phone number is what led the police to Muller's address, which, in turn, led to the overwhelming physical evidence against him in the Vallejo kidnapping. So if the courts rule that the 9-1-1 call was unconstitutional the "fruit of the poisonous tree" doctrine could theoretically result in most of the evidence in the case being thrown out. "The phone search was quite literally the door that led to a room full and a car full of evidence, not to mention the ultimate identification of Matt Muller as the possessor of the phone," Muller’s attorney, Thomas Johnson, writes in the motion. "All of this evidence must be suppressed to protect the integrity of the Fourth Amendment of the United States Constitution."

Douglas Rappaport, the lawyer for kidnapping victim Denise Huskins, says Huskins is not concerned with the motion. "We all know that Denise was a victim at this point. She went through a horrible, brutal experience, but she can't ultimately be concerned with what happens to him," he says. "For her to live or die by each decision the court makes would just destroy her.”

If Muller succeeds in getting the physical evidence removed, his “off-the-record” confession could prove critical to the prosecution. Some courts have ruled that reporters have a limited privilege to refuse to testify or turn over notes about off-the-record interviews, but that’s the extent of the protection afforded by the law. And as an inmate, Muller has no legally recognized right to privacy in his conversations with visitors.

"When you speak on the telephone or in interview rooms, there are signs, and you’re notified that visits are recorded," Rappaport says. "Speak at your own peril. Even if he goes off the record, that means that the reporter can't publish what he states. The feds are entitled to use whatever he says."