Lives might have been saved if Virginia Tech officials had acted sooner after student Cho Seung-hui's first killings, a state report into the shootings says.
"Warning students, faculty and staff might have made a difference," it says.
The independent panel also concluded that though Cho had demonstrated signs of mental instability earlier, college staff had not intervened effectively.
Cho killed 32 people and himself at the US university in April in the deadliest shooting rampage in modern US history.
The eight-member panel, appointed by Virginia governor Tim Kaine, said officials should have issued an alert or cancelled classes after Cho shot his first two victims - Emily Hilscher and Ryan Clark - in a dormitory just after 0700 on 16 April.
"The VTPD (Virginia Tech Police Department) erred in not requesting... a campus-wide notification that two persons had been killed and that all students and staff should be cautious and alert," the report said.
The campus police force initially suspected that Emily Hilscher's boyfriend was behind the shootings, believing it to be a domestic incident, and focused their efforts on finding him.
"Senior university administrators... failed to issue an all-campus notification about the West Ambler Johnston killings until almost two hours had elapsed," the report added.
More than two hours later Cho killed 30 students and teachers, plus himself, at the Norris Hall complex in another area of the campus.
At 0926 the university sent an e-mail to students and staff warning of a "shooting on campus" and urging caution.
However, while a lockdown might have helped protect some students and teachers it would have likely been ineffective in stopping Cho, who "had started on a mission of fulfilling a fantasy of revenge", the report said.
No-one knew all the information and no-one connected all the dots
Virginia Tech report
"From what we know of his mental state and commitment to action that day, it was likely that he would have acted out his fantasy somewhere on campus or outside it that same day," the report said.
The panel were also mindful of the fact that as a student himself, Cho would have had access to the same warning messages as everyone else and to the campus buildings.
In fact the report concluded that a complete lockdown of all 131 buildings in the Virginia Tech building was not feasible in the time available on the day, and that there was little the university could have done to secure the premises which would have halted Cho's attack.
"There does not seem to be a plausible scenario of a university response to the double homicide that could have prevented the tragedy of considerable magnitude on April 16," the report said.
However, the report was critical of how campus officials had handled signs of Cho's mental illness.
"During Cho's junior year at Virginia Tech, numerous incidents occurred that were clear warnings of mental instability," the report said.
Police were aware that Cho, who moved to the US with his family from South Korea in 1992, had been admitted to a mental health unit in late 2005.
He was sent for evaluation after two female students made complaints against him, following a period of bizarre behaviour and concerns that he was suicidal.
But the information was never passed on to university officials because of a lack of resources, misinterpretation of privacy laws and passivity, the report said.
"Although various individuals and departments within the university knew about each of these incidents, the university did not intervene effectively. No-one knew all the information and no-one connected all the dots," the report said.
In the lull between killings, Cho sent a package to US network NBC containing 28 video clips, 1,800 words of text and 43 photos, 11 of them showing Cho aiming handguns at a camera.
Although his motives remained unclear, the report traced his fantasies to the 1999 shootings at Columbine High School in Colorado, in which two students killed 13 people.
The state governor said the purpose of the inquiry was not seek to lay blame, but ensure that such a tragedy could not happen again.
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