MADISON, Wis. (AP) — A forensic psychiatrist said he believes Lou Gehrig’s disease is the reason a former Wisconsin sheriff’s deputy killed his wife and sister-in-law.

Dr. Doug Tucker testified for the defense Wednesday in the trial of Andrew Steele, who has pleaded not guilty by reason of mental disease in the August shooting deaths of his wife, 39-year-old Ashlee Steele, and her 38-year-old sister, Kacee Tollefsbol, of Lake Elmo, Minnesota.

Tucker said a message Steele wrote about suicide and sexual relations with his wife and sister-in-law was delusional and shows Steele’s brain was deteriorating because of ALS.

“This behavior in this guy… is a result of his frontal temporal major neurocognitive disorder,” Tucker said. “That’s the reason he killed his wife and her sister.”

Steele was “substantially unable to conform his conduct to the law” because of the disorder, Tucker testified.

Steele, a former Dane County deputy, was diagnosed with the progressive neurodegenerative disease last June.

The defense also called the victims’ sister, Hailee Meisterling, who testified about Steele’s behavior on a cruise the families took last year.

“He knew I was getting changed and he came into the room knowing I was getting changed into my swimsuit,” Meisterling said. “Another time I was in their room and he just started taking his pants off.”

Tucker and Meisterling testified in the second day of a trial that’s expected to last two weeks.

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