A police spokesman in suburban Atlanta says investigators may never know what motivated a FedEx employee to shoot six co-workers before fatally shooting himself.

Cobb County police Sgt. Dana Pierce said Thursday that a note left behind by the gunman, Geddy Kramer, doesn't shed light on his motive.

Pierce says statements from some of Kramer's coworkers initially led them to believe he might be retaliating against co-workers or the company. But Pierce now says the investigation hasn't turned up evidence to back that up.

Authorities said Thursday that a 28-year-old man shot at the facility remained in critical condition. Two others also remained hospitalized: a 52-year-old female woman in stable condition and a 22-year-old man in good condition. The three others have already been treated and released.