BLACKSBURG, Va. - The bloodbath lasted nine minutes - enough time for Seung-Hui Cho to unleash 170 rounds from his two pistols, or about one shot every three seconds.

A Virginia State Police officer patrols the campus of Virginia Tech as workers continue the process of putting a fence around Norris Hall on the campus in Blacksburg, Va., Wednesday, April 25, 2007. [AP]

During that time, Virginia Tech and city police spent three minutes dashing across campus to the scene. Then they began the agonizing process of breaking into the chained-shut building, which took another five minutes.

Once inside, as they sprinted toward the sounds of gunfire inside Norris Hall, Cho put a bullet through his head and died in a classroom alongside his victims.

A timeline of the rampage emerged Wednesday as police provided new details about what they uncovered in the 10 days since Cho committed the worst mass shooting in modern US history.

The five minutes police spent breaking into the building proved to be crucial. During that time, Cho picked off his victims with a hail of gunfire. He killed himself after police shot through the doors and rushed toward the carnage.

State police spokeswoman Corinne Geller praised the officers' response time, noting that had police simply rushed into the building without a plan, many would have likely died right along with the staff and students. She said officers needed to assemble the proper team, clear the area and then break through the doors.

"If you go in with your backs turned, you're never going back," Geller said. "There's gotta be some sort of organization."

Some police and security experts question the five-minute delay, saying authorities should have charged straight into the melee.

"You don't have time to wait," said Aaron Cohen, president of IMS Security of Los Angeles, who has trained SWAT teams around the country since 2003. "You don't have time to pre-plan a response. Even if you have a few guys, you go."

Cohen said that a trained SWAT team should have been able to get inside a locked building in less than a minute. There was no SWAT team at the Virginia Tech scene.

Police rapid response to school violence has become an important issue in the last decade.

After the Columbine massacre in 1999, police around the country adopted new policies for so-called "active shooters." Police would no longer respond to emergencies such as school shootings by surrounding a building and waiting for the SWAT team. Instead, the first four officers rush into the building and attempt to immediately end the threat. This system was used to end a 2003 school hostage standoff in Spokane, Wash.

Tom Corrigan, former member of the NYPD-

FBI terrorism task force and a retired New York City detective, said five minutes seems like a long time when gunfire is being heard, but he said it's tough to second-guess officers in such a chaotic situation.

"I would have liked to have seen them bust down the door, smash windows, go around to another door, do everything to get inside fast," he said. "But it's a tough call because these officers put their lives on the line on a daily basis and I am sure they did the best they could."

State Police Superintendent Col. W. Steven Flaherty, who is overseeing the investigative team looking at the shootings, said police have been unable to answer the case's most vexing questions: Why the spree began at the West Ambler Johnston dormitory, and why 18-year-old freshman Emily Hilscher was the first victim.

"We talk about possible motives and theories and whatnot, but we don't have any evidence to suggest anything," Flaherty said.

Witnesses place Cho outside West Ambler Johnston shortly before 7:15 a.m., when he fired the two shots that killed Hilscher and 22-year-old senior Ryan Clark, a resident assistant at the dorm, Flaherty said.

It is not known how Cho got in.

Police searched Hilscher's e-mails and phone records looking for a link. While Flaherty would not discuss exactly what police found, he said neither Cho's nor Hilscher's records have revealed a connection.

"We certainly don't have any one motive that we are pursuing at this particular time, or that we have been able to pull together and formulate," Flaherty said. "It's frustrating because it's so personal, because we see the families and see the communities suffering, and we see they want answers."

In addition to the 170 rounds Cho fired inside Norris, investigators found unused ammunition in the building, though Flaherty was unsure how much was left. Investigators have compiled 500 pieces of evidence from Norris Hall alone.

Virginia Tech police chief Wendell Flinchum said Cho had a class this semester in Norris Hall, although it was not scheduled to meet on the day of the rampage.

Flaherty cautioned that it could be months before the case is closed. The investigation will begin slowing down as authorities examine evidence, he said.