A male nurse was charged today with murdering 12 patients at two small rural hospitals in Riverside County.

Robert R. Diaz, 43, is accused of killing the patients with overdoses of the drug Lidocaine, which is used to treat heartbeat irregularities.

The Riverside County district attorney's office since last spring has been investigating the deaths of as many as 27 patients at Community Hospital of the Valleys in Perris and San Gorgonio Pass Hospital in Banning. Diaz was charged with drug possession four months ago after investigators found small amounts of Lidocaine and morphine in his home, but he was not officially named a suspect in the murders until today.

The publicity that followed the drug charge, however, including reports that Diaz was on duty at the time many of the patients died, led him to file a $300 million defamation-of-character suit against Riverside County officials. Subsequently, a municipal court ruled that the search of Diaz's house had been conducted under an improper warrant, and the drug charges were dropped.

Diaz's wife, Martha, said today: "As far as I know, my husband didn't do anything wrong, and I feel this is all to discredit the lawsuits and to get back for losing the issue of the search warrant the last time we were in court."

At a news conference this morning, an assistant Riverside County district attorney said that 11 of the 12 deaths in which Diaz is charged occurred at Community Hospital of the Valleys and one at San Gorgonio Pass. All the deaths occurred this year between March 30 and April 25 while Diaz was working early morning shifts, and all were the result of Lidocaine overdoses.

Diaz, who was assigned to the hospitals by a nursing registry on a temporary basis, acknowledged at the time the drugs were discovered at his home that he had administered Lidocaine to patients, but said he had done so only under physicians' orders or in accordance with standard procedure at the hospitals. He said he was "scared" by the high number of deaths in the hospitals' intensive-care units and insisted that he had tried to persuade officials at the hospitals to investigate their supplies and Lidocaine for signs of contamination.

Diaz claimed last May that county officials were trying to "railroad" him, and said he had refused to take a lie-detector test because investigators were tapping his phone.

Diaz is being held without bail. His arraignment is scheduled for Wednesday.