Juan Rivera sexually assaulted young Jadianna Larsen before killing her, and the man’s mother stole a sport-utility vehicle as part of a plan to cover up the girl’s death, Sacramento County prosecutors allege in a new complaint presented Friday in Sacramento Superior Court.

The disturbing development Friday was but the latest in the months since the charred body of the 6-year-old Sacramento girl was found miles from her south Sacramento apartment complex May 28, 2015, in a field in rural Glenn County. Sacramento County prosecutors say a blunt force instrument was used to kill Jadianna.

In the amended four-page complaint, prosecuting Sacramento County Deputy District Attorney Chris Ore added a special circumstance to the murder charge Rivera already faces, alleging the killing happened while Rivera performed a lewd act upon a child younger than 14 or that the girl died after he committed or tried to commit a lewd and lascivious act.

Juan Rivera, 25, was arrested May 29 on suspicion of killing the girl; and his mother, Lisa Burton, 47, was taken into custody as an accessory after the fact suspected of covering up the crime. On Friday, prosecutors also accused Burton of auto theft in connection with the killing, alleging she drove off with a 2002 Ford Expedition.

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Rivera and Burton remain held at Sacramento County Main Jail. Rivera is being held without bail; Burton remains in custody in lieu of $200,000 bail. Both are slated to return April 14 before Sacramento Superior Court Judge Michael Bowman, who is expected to set a date for a preliminary hearing in the case at the April hearing.

The case has shined a light on a brief, troubled life marked from birth by Child Protective Services files and reports of neglect – and her adult caretakers’ role in it.

One of those was Rivera, the boyfriend of Jadianna’s mother, Tanecia Clark. Rivera regularly stayed at Clark’s apartment complex with Burton and was asked to care for Jadianna while Clark was seeking mental health treatment, authorities and relatives said.

Rivera’s late-night call to emergency dispatchers May 28 that the girl had vanished brought authorities to the scene. Rivera told authorities he was stricken with seizures that lasted for hours and only learned Jadianna was missing when he recovered.

Earlier that day, 90 miles away, Glenn County sheriff’s investigators had discovered the body they would later identify as Jadianna’s.