In a surprise move, Tony T. Callihan pleaded guilty to second-degree murder Thursday, accepting a sentence of 20 years in prison.

Callihan, 40, was accused of strangling his girlfriend, 48-year-old Michelle Koenen, on Dec. 7, 2012. Police found him sleeping in Koenen’s car a week later in Clearlake, California, and he’s been in jail since.

A mistrial in his case was declared in December after the defense’s expert witness suddenly said he was unable to testify. There were also issues with 18 letters written to and from Callihan in prison that weren’t disclosed by prosecutors until the trial was underway.

A second trial was set to begin Monday, said Callihan’s attorney, Rob Cossey.

“He chose to take a plea,” he said. “I was very surprised. He hired me to go to trial.”

Callihan has a lengthy and violent criminal history, including pistol whipping a man at a party in 2001. Cossey said Callihan’s history meant that if he would have been found guilty at trial he could have been sentenced to nearly double the 20 years he accepted in the plea deal.

“He has a young son,” Cossey said. “He was very concerned about being able to spend some of his life with his son.”

Callihan will receive credit for the more than two years he has been in jail.