Aaron Schaffhausen, Wisconsin father, was erratic months before killing his three girls, defense lawyer says

(CBS/AP) HUDSON, Wis. - A Wisconsin father accused of killing his three young daughters was spiraling out of control in the months leading up to the girls' slayings last summer, his attorney said Tuesday.

PICTURES: 3 Wisconsin sisters dead; father arrested

Aaron Schaffhausen, 35, admitted to killing his daughters -- ages 11, 8 and 5 -- at their home in River Falls, Wis. But Schaffhausen is seeking to convince a jury that he's not responsible due to mental illness.

Defense attorney John Kucinski said during opening statements Tuesday that Schaffhausen became increasingly erratic and obsessive after he and his ex-wife, Jessica, divorced in January 2012. He said Schaffhausen called Jessica constantly and sometimes threatened to hurt her or the girls.

Prosecutors said Schaffhausen planned the killings and that contradicts his claim of mental illness.

Complete coverage of Aaron Schaffhausen on Crimesider