Investigators are "making some really great progress" into determining how and why the shootings happened, Virginia State Police spokeswoman Corinne Geller said Friday. She said they hope to have something to tell the public next week.
"The why and the how are the crux of the investigation," Geller said. "The why may never be determined because the person responsible is deceased."
Cho fired between 175 and 225 rounds of ammunition, law enforcement sources told CBS News. The majority of the shots were from a 9 mm Glock.
One law enforcement source told CBS News: "Once the shooting starts, there are always more shots fired than people think. That's kind of the nature of the beast."
Authorities want to know why Cho selected Hilscher and Ryan Clark as his first victims. "We're trying to establish those links, if there are any," Geller said.
The governor also appointed an independent panel that includes former Homeland Security Secretary Tom Ridge to look into how authorities handled the tragedy.
Ridge said the group would look into the time lapse between the first attack and the second, how students were notified of the dangers, and whether privacy laws and the need to communicate for safety conflicted, among other things.
"This was out-and-out murder," Ridge said. "This was a horribly, horribly deranged young man."
Asked if the attacks had been preventable, Ridge said his "preliminary judgment would be probably not," but he said he hoped the investigation would find ways to reduce the risk in the future.
"This is a national tragedy, and we have to learn some lessons and apply them," Ridge told CBS' The Early Show. "The last thing we want to do in our universities and public places is turn them into citadels or armed fortresses."
Cho's videos, which were mailed to NBC the morning of the killings, revealed a man angry at the world but offered little explanation of why, other than rambling tirades against rich kids, snobs and people who had wronged him.
As experts analyzed the disturbing materials, it became increasingly clear that Cho was almost a classic case of a school shooter: a painfully awkward, picked-on young man who lashed out with methodical fury at a world he believed was out to get him.
for an interactive gallery of the victims.
"In virtually every regard, Cho is prototypical of mass killers that I've studied in the past 25 years," said Northeastern University criminal justice professor James Alan Fox, co-author of 16 books on crime. "That doesn't mean, however, that one could have predicted his rampage."
Among other things, the South Korean immigrant was sent to a psychiatric hospital and pronounced an imminent danger to himself. He was accused of stalking two women and photographing female students in class with his cell phone. His violence-filled writings were so disturbing he was removed from one class, and professors begged him to get counseling.
Classmates in Virginia, where Cho grew up,, apparently because of shyness and his strange, mumbly way of speaking.
Among the victims of the Virginia Tech massacre were two other Westfield High graduates, Reema Samaha and Erin Peterson. Both young women graduated from the high school last year, but police said it is not clear whether Cho singled them out.
In other developments: