SAN JOSE >> Matthew Muller, whose notorious 2015 kidnapping of a Vallejo woman inspired the Netflix documentary “American Nightmare,” has pleaded guilty in two Santa Clara County home invasions six years earlier, following a renewed investigation spurred by his prison letters to a California police chief.

Muller, 47, has been held in the Main Jail in San Jose since Dec. 27 after he was transferred from a federal prison in Tucson, Arizona, after being convicted in the Vallejo kidnapping. He was arraigned Dec. 30 on two felony charges of assault with intent to commit rape during a first-degree burglary.

Friday, he admitted to committing two home invasions in 2009 during which he held captive, interrogated and threatened to rape two women in separate instances in Palo Alto and Mountain View.

Muller appeared in a San Jose courtroom where he affirmed the two guilty pleas with Judge Cynthia Sevely. His only statements in court were variations of answering, “Yes, your honor” and “I do” when Sevely asked him a series of questions to confirm that he was entering plea voluntarily.

Matthew Dengel, the deputy public defender representing Muller at the hearing, told Sevely “the pleas are over my advice, your honor.”

Deputy District Attorney Brian King said he is “hopeful that this conviction today brings healing and closure to the victims of his crimes.”

“We further commend their endurance and their strength in light of the 15 years that they’ve awaited justice for what happened to them,” King said.

Muller is already serving a 40-year federal prison sentence and a concurrent 31-year state prison sentence related to the 2015 kidnapping and rape of Denise Huskins Quinn after he surveilled then broke into the Vallejo home she shared with her now-husband Aaron Quinn. The case attracted international attention after Vallejo police wrongly called the kidnapping a hoax, only for Muller himself to contact a news reporter to corroborate the crime.

The minimum sentence for each of Friday’s convictions is seven years to life in prison and must be served on top of any existing state sentence because they involve separate instances of sexual violence. That means after he completes the 31-year term, he will then serve the two life terms, which pushes his total incarceration past the 40-year boundary dictated by his federal sentence.

Muller is now being investigated for similar attacks elsewhere in Northern California from the past three decades and has been charged with a kidnapping in San Ramon from the same timeframe as the Vallejo crimes.

With the support of Huskins Quinn and Quinn, Nick Borges, police chief for the Monterey County city of Seaside, struck up a letter-writing exchange with the imprisoned Muller, leading to correspondence that amounted to confessions to the home invasions in the South Bay.

About a week after he was charged with those crimes, Contra Costa County authorities announced that they were charging him with a 2015 kidnapping in San Ramon that reportedly took place two weeks after the Vallejo case but which only surfaced in recent months after Muller implicated himself in the letters.

Authorities are investigating suspicions that Muller’s criminal patterns date as far back as 1993, when he was 16, and could involve as many as a half-dozen total kidnappings.

The first South Bay attack Muller admitted to committing occurred Sept. 29, 2009, when he allegedly broke into a woman’s Mountain View home, with the victim telling police that she woke up to a man in a ski mask pushing her face down in her bed and telling her he was committing an identity theft robbery.

The intruder handcuffed her and bound her ankles with “some sort of Velcro restraint,” made her drink Nyquil and used her phone to make several calls and also send text messages to her employer stating that she was calling in sick. The woman said the man stated his intent to rape her but that she was able to persuade him to change his mind and flee.

A few weeks later, on Oct. 18, Muller broke into a Palo Alto home and ambushed a sleeping woman while wearing a mask over his head. A police report states that she described a man speaking in a “low growl” — as if he was “knowingly trying to disguise his voice” — while restraining her with fabric fasteners on her ankles and arms, putting plugs in her ears and covering her eyes with surgical tape.

The woman was also made to drink Nyquil. At some point, the man also stated his intention to rape her before she told him about a past sexual assault, which apparently caused him to relent. He then warned her against calling the police and left.

Muller was considered a suspect early on, but DNA analysis was inconclusive. In April and May, Borges reportedly received letters from Muller in which he volunteered information implicating himself in the 2009 home invasions.

Borges shared that information with Palo Alto police, prompting a renewed investigation that reexamined DNA traces from the fabric fasteners used in the home invasions and matched Muller to the two Santa Clara County cases. King said he and detectives from Palo Alto and Mountain View police traveled to Arizona to interview Muller in prison as part of the investigation.

King lauded the contributions of Huskins Quinn and Quinn, saying their efforts to help police learn from their experiences ultimately opened up new avenues to link more cases to Muller.

“Denise Huskins Quinn and Aaron Quinn were victimized by Matthew Muller in 2015, and they were then re-victimized by an unacceptable police investigation,” King said. “They’ve used that experience and turned it into a force for good. … This would not have happened without their advocacy and their pursuit of justice.”

Staff writer Jakob Rodgers contributed to this report.