A 26-day investigation has unearthed much about the troubled life of the woman who killed a schoolboy and wounded six other people during a spree of violence, but police may never know the motive behind her actions.

A task force of officials from the Winnetka Police Department and 10 other law enforcement agencies ruled out accomplices and the influence of prescription pyschiatric drugs and said the May 20 shooting spree in the affluent Chicago suburb was not the result of an imbalance in her body chemistry.

It was early this year that Laurie Dann, 30, began planning her violent binge in which she also sent 26 packages of arsenic-tainted food and juice to locations in three states, police said. At least five people were hospitalized after consuming the spiked treats, but none was seriously injured.