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A shackled Matthew Muller entered the plea in federal court in Sacramento. He politely and calmly answered a judge's questions about the crime and the rights he was waiving by pleading guilty.
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U.S. District Court Judge Troy Nunley said he found Muller competent to enter the guilty plea.
Muller previously pleaded not guilty to abducting Denise Huskins in March 2015. Her boyfriend, Aaron Quinn, said kidnappers broke into the couple's Vallejo home, took Huskins and demanded $8,500 as a ransom- a figure that police have said they found small for what would have been an elaborate kidnapping.
Huskins turned up safe two days later in her hometown of Huntington Beach, where she says she was dropped off. After she reappeared, Vallejo police called the kidnapping a hoax.
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Huskins sued, accusing police of wrongly likening the case to the movie "Gone Girl" and damaging the reputations of her and her boyfriend.
Attorneys for police have said investigators doubted Quinn's account of the abduction and grew more skeptical when Huskins refused to reunite with her family soon after she reappeared.
Muller was later charged. A call to his attorney, Thomas Johnson, was not immediately returned.
Johnson has said Muller has bipolar disorder. In a court filing this week, prosecutors asked a judge to inquire during Thursday's hearing about Muller's mental condition and medications to make sure he understands the proceedings and his rights.
Muller was admitted to practice law in California in 2011, and his state bar profile says he attended Harvard Law School.
He lost his law license last year over allegations that he took a $1,250 advance from a client then failed to file a green card application for the person's son.
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BREAKING: Matthew Muller suspect in Vallejo kidnapping is entering guilty plea, accepting deal. pic.twitter.com/HrQ9CHjr3K— Melanie Woodrow (@MelanieWoodrow) September 29, 2016
Plea agreement terms: guilty to kidnapping, will pay any fine, govt will not pursue any additional crimes or term in excess of 40 yrs— Melanie Woodrow (@MelanieWoodrow) September 29, 2016
Govt asks that Muller be subject to most intensive supervision and monitoring avail at release— Melanie Woodrow (@MelanieWoodrow) September 29, 2016
Muller appears pale, wearing dark rimmed glasses/orange jumpsuit, answering judge coherently, has described meds he's on— Melanie Woodrow (@MelanieWoodrow) September 29, 2016