Four months into their investigation of the Virginia Tech shootings, authorities still have no idea what motivated Sueng-Hui Cho’s transformation from campus recluse to mass murderer.

Although much remains unknown, police held a news conference Friday to announce several new details:

-- Two days before the April 16 shootings, a witness spotted a "suspicious-looking male" in a hooded sweatshirt inside Norris Hall. Another witness saw a set of doors chained shut at about the same time — in much the same way three entrances were blocked the day Cho shot 30 people and then himself inside.

-- Earlier that morning, Cho left a footprint in a hallway at West Ambler Johnson Hall dormitory, providing the most conclusive evidence to date that he started his killing spree by fatally shooting students Emily Hilscher and Ryan Clark in Hilscher's dorm room.

-- That evidence was bolstered further when police found Clark’s blood on Cho’s blue jeans, which the gunman changed out of and left in his dorm room before launching his attack on Norris Hall.

These latest details shed no light on the larger mystery.

"At this stage, we still have no evidence that answers the persistent questions: Why West Ambler Johnston? Why room 4040? Why Emily Hilscher?" said Virginia Tech Police Chief Wendell Flinchum.

"We just don’t know."