( CNN ) -- A convicted sex offender who was sentenced to death for killing a 9-year-old Idaho boy is scheduled to be arraigned Monday on charges related to the killing of a 10-year-old California boy . Joseph Edward Duncan III will answer charges in the 1997 abduction and murder of Anthony Martinez . Joseph Edward Duncan III is expected to appear in court in Riverside County , California , to answer to charges in the 1997 abduction and murder of Anthony Martinez , said a spokesman for the Riverside County District Attorney 's office . Investigators have charged Duncan with murder in Anthony 's death . Law-enforcement officials also are investigating whether Duncan , who committed his first sexual offense at the age of 12 , can be tied to other crimes , according to CNN affiliate KTLA in Los Angeles , California . Martinez was kidnapped on April 4 , 1997 from an alley near his home in the city of Beaumont , the station reported ; the boy 's naked body was discovered a little over two weeks later . Duncan has confessed to killing Martinez and crushing the boy 's head with a rock , KTLA reported , citing court documents . Authorities reportedly discovered the boy 's body by following vultures to a remote section of a nearby canyon . The body was partially buried under a pile of rocks and bound with duct tape , according to KTLA . Duncan , a high school dropout and drifter , was sentenced to death last August for the torture and murder of Dylan Groene , a 9-year-old Idaho boy . He was convicted of kidnapping Dylan and his then 8-year-old sister before torturing them at a remote campsite and fatally bludgeoning members of their family . In 2007 , Duncan pleaded guilty to murder and kidnapping counts in state court for the hammer-attack murders of three other Groene family members . If convicted of murdering Martinez , Duncan can be sentenced to death again .
David Duncan committed his first sexual assault at the age of 14 . He was sentenced to death last August for the killing a 9-year - old boy . He could be sentenced to death a second time if convicted again .