HUDSON, Wis. -

Former coworkers of Aaron Schaffhausen have testified they heard him say several times that he wanted to kill his ex-wife, their children and the man she was dating in the months before he killed his three young daughters.

The St. Croix County Circuit Court jury considering his case heard from the former coworkers from Minot, N.D., on Friday.

Schaffhausen pleaded guilty last week to three counts of first-degree intentional homicide for killing his three daughters in their River Falls home last July.

He contends he was not responsible for the killings because of mental illness. But prosecutors maintain he was sane, and that he killed the girls to take revenge on his ex-wife because he was angry with her for divorcing him and for seeing another man.