I killed 50 people, says 'Angel of Death'.
But Efren Saldivar, who admitted an eight-year killing spree, remains free as police try to discover if the murders actually took place.
"Hospital files will have to be searched and bodies might have to be exhumed," said a police spokesman.
The killings started in 1989 at the Glendale Adventist Medical Centre, where Saldivar worked as a respiratory therapist.
The self-styled Angel of Death told police he had a three-point checklist for "candidates" for his "mercy killings".
They had to be unconscious; there had to be a "do not resuscitate" order on their medical chart; and they had to look "as if they were ready to die".
He killed them with a lethal injection of paralysing drugs or by reducing the oxygen supply.
Police launched their investigation after an anonymous tip-off.