Four months into their investigation of the Virginia Tech shootings, authorities still have no idea what motivated Seung-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:

Cho's detailed planning might have included a practice run. Two days before the April 16 rampage, a witness spotted a "suspicious-looking male" in a hooded sweat shirt inside Norris Hall. Another witness about that time saw a set of doors chained shut -- in much the same way three entrances were blocked the day Cho killed 30 people and then himself inside.

Early on April 16, Cho left a bloody footprint in a hallway at West Ambler Johnston 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 jeans and shoes, which the gunman hid in his dorm room before launching his attack on Norris Hall.

Yet the latest details gave no hint of Cho's motives.

"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."

Police are still looking for several missing links -- including Cho's computer hard drive and his cellphone -- that might explain a motive.

With no end to the investigation in sight, authorities are unable to say for sure that the hooded man seen in Norris was Cho conducting a practice session two days before the shootings.

But the sighting was considered significant enough for police to include it among the few new details released Friday at a news conference in Roanoke.

Other details reinforced what authorities have already said about Cho, a loner whose bizarre behavior and dark writings for an English class raised concerns on a campus where few people knew him by name before April 16.

Handwriting analysis confirmed that it was Cho who scrawled the words "Bomb will go off if you open the door" on a note that was taped to a door in Norris. Faculty member Janis Terpenny found the note and gave it to janitor Pamela Tickle just minutes before the shooting started.

Terpenny and Tickle later said they believe Cho posted the warning to discourage people from entering a hallway and interrupting him as he chained and padlocked the doors.

While handwriting analysis linked Cho to the bomb threat, it also excluded him from making several bomb threats on campus earlier in the month, Flinchum said.

After the shooting at West Ambler Johnston, Cho returned to his room at Harper Hall about 7:20 a.m., changed clothes and deleted his personal e-mail account, police said.

He also mailed a multimedia package to NBC News in New York, acknowledging his role in the shootings. At the same time, he mailed a rambling letter to Tech's English department.

Cho's writing was incoherent and offered no insight into the shootings, Flinchum said.

About 8 a.m. -- after the Ambler Johnston shootings but before the ones at Norris -- a student reported seeing an Asian male walking near the Tech Duck Pond. Police searches of the pond for Cho's hard drive turned up nothing.

Later that morning, Cho was seen peering into several classrooms at Norris.

Witnesses said he was wearing a black T-shirt, cargo pants and a Virginia Tech baseball cap. On his left arm, he had written the words "Ax Ishmael" in red ink. Police still have no clue what those words mean.

A short time later, Cho walked into the rooms one by one, methodically shooting students and professors.

"Not saying a word, he went room to room and returned to some of the rooms, firing again," state police Superintendent Steve Flaherty said.

The rampage lasted about nine minutes. As police closed in, Cho -- who by then had fired more than 170 rounds and had 203 left -- shot himself in the head.

In addition to the 30 fatalities in Norris Hall, Cho wounded 23 people. Two more people were injured as they jumped from windows, Flaherty said. The news conference marked the first time police have given a detailed accounting of the wounded.

Few details emerged Friday about Cho's psychiatric problems and his brush with the mental health system 16 months before the shootings. The 23-year-old English major was held overnight in a mental hospital after his roommate reported that he might be suicidal, but a special justice released him the next day on orders to receive outpatient treatment.

Questions remain about whether Cho received follow-up treatment after the December 2005 hearing.

Flaherty said Cho had "multiple contacts" with mental health facilities, but declined to go into details.

A more complete account of how Cho might have slipped through the cracks of the mental health system is expected when a panel appointed by Gov. Tim Kaine releases its findings later this month.

Police said it could be months before they wrap up the investigation.

"We're investigating 32 homicides, a suicide and 23 attempted homicides," Flaherty said. "That type of investigation, by its very nature, takes a long, long time. You simply can't put a deadline or a limit on the time it takes to conduct the investigation."