Los Angeles police are excavating around two Ventura County freeway offramps for the remains of a boy believed to have been killed by a serial murderer 40 years ago.

Roger Dale Madison was last seen alive in December 1968. A serial killer linked to as many as 18 kidnappings and murders in the 1950s and 60s confessed that Madison was one of his victims. Mack Ray Edwards claimed he buried the boy near the 23 Freeway. He later hung himself while on death row at San Quentin.

It's not clear why police did not look into Edwards' confession. The case was revived when a writer looking into a 1957 missing child case noticed similarities with the Madison case.

Officers, who started digging Monday, hope to find Madison's remains and give them to his siblings, who say they plan to have them cremated and buried with the ashes of the boy's parents.