A former hospital nurse pleaded no contest Tuesday to killing 10 patients nearly six years ago by injecting them with a drug used to temporarily halt breathing.

Vickie Dawn Jackson, 40, will be sentenced to life in prison, the same sentence she faced if she had been convicted by a jury in San Angelo.

Authorities have not offered a motive for the slayings.

Defense attorney Bruce Martin said Jackson decided to enter the plea because her adult daughter was on the state's witness list.

"She has never admitted guilt and she was never convicted by a jury," Martin said. "And her daughter never had to testify against her. Those things meant something to her."

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Jackson was accused of killing the patients, including her third husband's grandfather, by injecting them with a drug used to stop breathing to allow insertion of a breathing tube.

Prosecutor said the deaths occurred during her night shifts at Nocona General Hospital in 2000 and 2001. More than 20 vials of the drug were missing and a syringe with traces of the drug was found in the nurse's garbage, they said.

Prosecutors were surprised by the plea, which came less than a week before Jackson's trial was scheduled to begin.

"Frankly, I've never been so surprised in a case in my life," said Jack McGaughey, district attorney for Montague, Clay and Archer counties, who had planned to call 58 witnesses. "The end result is as good as we could have gotten."