HUDSON, Wis. — A father changed his pleas Thursday to guilty but insane in the killings of his three young daughters last summer in Wisconsin, which means the defense at his trial will have to prove he had a mental disease or defect so severe that he wasn’t responsible for his actions.

Aaron Schaffhausen, 35, had pleaded not guilty, but switched his plea after more than a day of delays and legal wrangling about what kind of evidence will be allowed at his trial, which starts next week.

Schaffhausen spoke in a flat tone that lacked emotion as he answered the judge’s questions about his new pleas.

“Are you pleading guilty because you are guilty?” Judge Howard Cameron asked at one point.

“Yes,” Schaffhausen responded.