(CNN) The crime Texas prosecutors proved three decades ago was horrifying: A nurse entrusted with caring for young children killed a 15-month-old girl by injecting her with powerful drugs, they said.

Genene Jones was sentenced to 99 years in prison. Then authorities discovered she might be released next May, after serving less than a third of her term, because of an old law designed to relieve prison overcrowding.

Dismayed, they began investigating Jones anew, saying dozens of children had died in her care.

Thursday, new charges were filed.

Jones, now 66 and still in the prison system, was indicted on a charge of murder in the December 12, 1981, death of 11-month-old Joshua Sawyer. If convicted, she could spend the rest of her life in prison.

Bexar County District Attorney Nico LaHood said evidence shows she killed the child with a toxic level of Dilantin while he was a patient at Bexar County Hospital in San Antonio, now called University Hospital.

LaHood told CNN his office "got fresh eyes" to review old evidence from the original case in the 1980s and discovered new evidence in the process. Sawyer's mother held onto medical records and shared them with investigators, LaHood told CNN affiliate KSAT

He didn't offer any specifics about the motive.

"As people are well aware, I believe children are a gift from the Lord," LaHood said. "Genene Jones did not see children in this regard. She is pure evil and justice warrants that she be held accountable for the crimes she committed."

Infant deaths spiked

LaHood said Jones worked for about four years at hospitals in the San Antonio area. The pediatric units showed a sharp spike in infant deaths during the shifts Jones worked, normally 3 to 11 p.m., said Cheri Pendergraft, a former pediatric nurse who used to work with Jones.

During the investigation, LaHood's office determined Jones could be responsible for the deaths of nearly 60 children -- a number investigators came to by looking at the children who died while she was on shift at the hospitals.

They also considered children who died just after her shifts ended.

LaHood didn't say if any other charges would be filed.

Heart-stopping drug

Jones was first convicted in 1984 in the death of Chelsea McClellan. She was 15 months old in 1982 when her mother brought her to a local clinic in Kerrville, Texas, about an hour away from San Antonio.

"She gave her her first shot in her left thigh and she immediately started gasping for air," Petti McClellan-Wiese told CNN during a 2013 interview. "Gave her another one, and she immediately just went limp and quit breathing."

In the chaos of rushing Chelsea from the clinic to the hospital, Jones somehow slipped into the ambulance and gave the little girl a third shot.

McClellan-Wiese would learn later that the nurse had injected her daughter with a drug called succinylcholine, which causes muscle relaxation and short term paralysis. It stopped Chelsea's heart.

Jones pleaded not guilty and insisted she had nothing to do with the deaths.

She was convicted in Williamson County of infanticide and sentenced to 99 years in prison for killing Chelsea.

Later in 1984, she was prosecuted in Bexar County for injecting Rolando Santos, then 4 weeks old, with a lethal dose heparin and sentenced to 60 years in prison, LaHood's office said. The baby survived.

The two sentences were to be served concurrently.

Bond set at $1 million

LaHood said the Jones case had bothered the San Antonio community for years, especially when people learned she might be released after serving only 34 years of the 99-year term because of the Mandatory Release Law.

That law allowed inmates convicted of violent crimes between 1977 and 1987 to be automatically released if their "good behavior" credit plus their time served equaled their sentence. The law was changed in 1987 to exclude violent criminals, but it isn't retroactive.

Jones might would have been released from prison in May 2018, the prosecutor said.

"It always shocked people that this woman would serve a third of her sentence," LaHood said.

It's not known if Jones has an attorney for the new charge.

Jones is being held at a prison in Gatesville, Texas, and will be brought to Bexar County next year, LaHood said. Bond on the new charge was $1 million.