Authorities were investigating the death of a patient who was given a "non-prescribed dosage" of a medical drug by a replacement hired when thousands of nurses went on strike across California, an Oakland police spokeswoman said Sunday.

Spokeswoman Johnna Watson did not release details of the incident at Alta Bates Summit Medical Center. The patient died Saturday, two days after the nurses walked out during the labor dispute on Thursday. Many were not allowed to return to work on Friday.

Steve O'Brien, a doctor and the hospital's vice president of medical affairs, said the death was likely caused by a medical error. The patient had been receiving treatment at the hospital since July.

The lockout, and concerns about patient care provided by replacement nurses, prompted officials with the California Nurses Association to file a complaint with the state Department of Public Health, according to Charles Idelson, a spokesman for the union.

"We were concerned if the nurses they brought in during the lockout had the demonstrated competency and certifications required by California law," Idelson said.

A call to the Department of Health seeking information about the union's complaints was not immediately returned Sunday.

Hospital officials disputed claims by the nurse's union that the replacement nurses were not qualified.

"Every single one of the nurses is an experienced nurse that has been working in the areas to which they are assigned," said O'Brien. "We did not skimp on any of the nurses."

No details about the patient or the sort of treatment being administered were released because of privacy laws. The Oakland Tribune reported that the patient was a 66-year-old Oakland resident.

Carol Weis, chief nursing executive at the hospital, said in a statement that the death "is a tragic event and our heart goes out to the family of the patient."

"While medical errors do exist in health care, we are constantly investing in ways to improve patient care," O'Brien said. This is a most unfortunate event for which we are very sorry."

The nurses union planned a candlelight vigil at the hospital, with nurses calling on Sutter Health, the operator of Alta Bates, to end the lockout and for the state to investigate what union officials called "safety violations" in the patient's death.

About 23,000 nurses across California walked off the job Thursday in a one-day strike at 33 not-for-profit hospitals run by Kaiser Permanente, Sutter Health and the independent Children's Hospital Oakland. They had planned to go back to work Friday, but were locked out when they tried to return to work at Sutter hospitals, according to union officials.

Sutter officials said they entered into contracts with replacement nurses that required they pay the replacement nurses for a minimum number of days.

"Once a strike is called, it would be financially irresponsible for hospitals to pay double to compensate both permanent staff and replacement workers," Sutter Health said earlier in the week.

The lockout was scheduled to end Tuesday.