CHICAGO (CBS) — The family of an elderly woman who died of an overdose of morphine last year has filed a wrongful death lawsuit against the nursing home where she lived.

Dorothy Byrd, 98, was one of six seniors rushed to the hospital from Holland Home in South Holland on Feb. 3, 2015, after they were allegedly sedated by a nurse.

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The lawsuit — filed in Cook County Circuit Court against Holland Home and nurse Talisha Lillard — alleges Lillard the only worker at the nursing home that night who could unlock the drug cabinet.

A police source with knowledge of the investigation has told Newsradio that it was believed that the patients were sedated to lighten the workload while the facility was understaffed on the day after a snow storm.

Byrd, a retired nurse, died the next day, and two others died about a month later, although Byrd’s death was the only one determined to be the result of an overdose. The other two deaths were ruled to be from natural causes.

An autopsy determined Byrd died of toxic levels of morphine and hydrocodone, and her death was ruled a homicide. An attorney for Byrd’s family has said she was taking hydrocodone for back pain, but was not prescribed any morphine.

Byrd’s daughter, Sandra Peterson, said it’s troubling South Holland police, the South Suburban Major Crimes Task Force, and the Cook County State’s Attorney’s office have yet to file any criminal charges in the case.

Peterson, who is also a nurse, said negligence at the nursing home led to her mother’s murder.

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“I’m very sure that we will establish that. Through my review of certain facts related to this case, it’s fairly evident to me that there was inadequate record-keeping in terms of controlled substances,” she said. “I’ve never seen anything like this before.”

She also wants the Illinois Department of Public Health to step in and revoke licenses.

Peterson said she had made a decision a week before her mother’s murder to move her to another nursing home because of numerous problems, but “I just ran out of time.”

Lillard and two certified nursing assistants have been fired, but no criminal charges have been filed.

In a videotaped deposition last year, Lillard denied any involvement in Byrd’s death.

“I do not know how she received morphine,” Lillard said.

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Holland Home said it’s cooperating with the ongoing police investigation. Police would only say the case remains open.