The victim’s mother, a former special-education teacher in the Renton School District, admitted to helping cover up the murder of her 18-year-old son and was convicted of rendering criminal assistance.

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A 45-year-old Auburn man who was convicted of torturing and killing his ex-girlfriend’s severely autistic 18-year-old son was sentenced Friday to 65 years in prison.

Matt Christenson was convicted by a King County jury in August of homicide by abuse in the death of Otto Smith, who died in April 2014. The defendant was also convicted of three counts of second-degree assault-domestic violence for beating Smith’s younger brother and one count each of felony harassment and unlawful imprisonment.

The charges included aggravators based on Christenson’s ongoing pattern of abuse, his deliberate cruelty and the victims’ vulnerability, according to the prosecutor’s office. Prosecutors had recommended the 65-year sentence based on the aggravators, far beyond the standard range of 30 to 41 years.

The victims’ mother, Pascia Backman, met Christenson through an ad posted on Craigslist in December 2013, charging papers say. He moved into her two-bedroom Auburn apartment along with his then 20-year-old son.

Beginning in January 2014, Smith and his brother, then 14, were subjected to beatings, forced exercise and hourslong ice baths, say the charges, which also accuse Christenson of withholding food from the brothers, shaking cayenne pepper into their eyes and zapping Smith with a stun gun.

Smith was also forced to wear garbage bags as diapers, and the younger boy was required to sleep on the bathroom floor.

According to the charges:

Christenson pulled Smith out of an ice bath, bound his legs and arms, shoved a sock into his mouth and wrapped his head in duct tape. Backman found him underneath a large sofa chair while Christenson sat a few feet away, drinking beer and smoking marijuana.

Backman removed the chair, then unwrapped the duct tape and took the sock out of her son’s mouth — but Smith “took his last breath” while lying on the living-room floor, according to the charges. She washed the tape residue from Smith’s face, then she and Christenson drove Smith to a hospital, where the mother told staff Smith had fallen down some stairs, the charges say.

He spent several days in the hospital, then another two weeks at a children’s respite-care facility in Yakima, according to the charges.

Smith was released from the care facility on April 10, 2014, but died two days after returning to the family’s apartment, the charges say.

Three days before Christmas 2014, Backman arrived by ambulance at Tacoma General Hospital, claiming to be a kidnap victim before admitting to police that she was trying to flee an abusive relationship with Christenson. She told a social worker Christenson had killed Smith and that she had helped him cover up the crime, according to charges.

The social worker reported the information to police, and the King County sheriff’s Major Crimes Unit opened an investigation into Smith’s death.

Backman, a former special-education teacher in the Renton School District, pleaded guilty in November 2015 to first- and second-degree rendering criminal assistance.

She wasn’t sentenced to jail time but was ordered to undergo two years of mental-health and substance-abuse treatment, with her progress monitored by the court, records show. Backman was also ordered to have no contact with her younger son, who is now 17.