Robert Diaz had always proclaimed to want to be a doctor. He eventually settled for nursing. This apparently wasn't a powerful enough position for Diaz. In April of 1981 an anonymous call was placed to the San Bernadino County coroner claiming that Diaz had possibly killed as many as 19 persons at a hospital in Perris, California. An investigation was immediately begun and it wasn't long before it was obvious that Diaz was responsible for the deaths.

Diaz had been the nurse on duty when all the patients died, which was only a 12 day period. He repeatedly predicted the time that some patients would die, almost precisely. He was even witnessed giving some of the same patients injections prior to their death. Medical records of some of the intensive care patients were missing, and Diaz had access to these records. He worked just one night at San Gregorio Pass Hospital but a patient had died there that night under the same circumstances as the victims in the Perris Community Hospital.

Diaz was arrested in November of 1981 and charged with the deaths of twelve patients. His trial did not take place until March 1984. Samples taken from exhumed bodies showed overdoses of the drug Lidocaine, which was also found in Diaz' home along with syringes and quantities of morphine. He was easily convicted on March 29, 1984 and sentenced to die.