Authorities have dropped a second charge against a Bismarck woman accused of abusing and exploiting her elderly mother during protests against the Dakota Access oil pipeline in North Dakota.

Authorities last week dismissed a charge of felony exploitation of a vulnerable adult against Kathleen Bennett, saying the case had become too difficult to prove, The Bismarck Tribune reported . A defense attorney said Bennett's mother died a few months ago.

Bennett, 59, was accused of leaving her 82-year-old mother with dementia tied to a chair in a protest camp in North Dakota while she attended demonstrations in December 2016. Protesters were trying to block construction of the oil pipeline, which is operated by Dallas-based Energy Transfer Partners.

Bennett's mother was taken to a hospital during a blizzard. Hospital staff said she was frail and malnourished.

The exploitation charge resulted from Bennett allegedly using $1,200 of her mother's money without consent to rent hotel rooms, buy meals and pay legal fees while her mother was hospitalized.

Bennett had also been charged in Morton County with endangering a vulnerable adult, but the defense and prosecution agreed in November to dismiss that case with $2,050 in fines forfeited from Bennett's bond.

"The victim is deceased and the case became difficult to prove once the Morton County companion case was dismissed," Burleigh County Assistant State's Attorney Marina Spahr said in court documents.

It's not clear when her mother, Mary Trujillo, died. She had been living with family in Nevada. Defense attorney William Kirschner said Trujillo's death happened a few months ago, but he did not have an exact date.

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Information from: Bismarck Tribune, http://www.bismarcktribune.com