BLACKSBURG, Va. - Four and a half years ago, the sound of horns and alarms did not have the same effect on Jerzy Nowak.

On the morning of April 16, 2007, the Virginia Tech horticulture professor was at his desk in Sanders Hall, writing research grant proposals, when the wail of sirens split the crisp, snowy air. It would be hours before he learned that a gunman was loose on campus - and more than half a day would pass before university officials arrived at his home to confirm that his wife, French professor Jocelyne Couture-Nowak, was among the 32 slain.

So when the alarms again sounded Thursday morning, Nowak was immobilized.

"I froze ... like my life just stopped for a split second," he said in his thick Polish accent. "I actually reacted ... much stronger than last time. Because, last time, I was not aware what was going on."

For those who were there in 2007, the shooting death of a campus police officer in a school parking lot and the apparent suicide of his killer brought back harrowing memories of what remains the worst mass killing in United States history. But the police and university response to the incident were also proof that the painful sacrifices of "4/16" were not in vain.

"There have been a lot of people that have been working REALLY hard since 2007 to make sure that there are systems in place to prevent what happened in 2007 and the miscommunications that could have saved people's lives from EVER happening again," said Fred Cook, 26, who broke his ankle jumping from a second-floor window of Norris Hall, where student Seung-Hui Cho killed 30 of his victims and eventually took his own life.

"So it pays tribute to the work that some of the survivors and some of the groups that have come out of this have done in that time, what the university has done in that time, to make sure that this doesn't happen again."

Officer Deriek W. Crouse was making a traffic stop when police say Ross Truett Ashley, a 22-year-old student at a nearby college, walked up to his cruiser and shot him. Within minutes of the 911 call, an alert went out and the campus was locked down.

Cho shot his first two victims around 7:30 a.m. on the fourth floor of West Ambler Johnston Hall. Campus officials, believing this was a domestic shooting, didn't lock down the campus or issue an alert.

It wasn't until 9:26 a.m. that the university sent out the first email to students and faculty. The subject line read, "Shooting on campus." But it wasn't until nearly a half hour later that a second email warned everyone to "stay put" and said a gunman was on the loose on campus.

Coincidentally, this latest shooting occurred the same day university officials were in Washington to appeal a $55,000 fine by the Department of Education for delays in notification during the 2007 rampage.

Derek O'Dell thinks an alert immediately after the first shootings four years ago might have changed his life.

That day, O'Dell arrived on campus around 8 a.m. for an exam. He was blissfully unaware that anything was amiss as he strolled over to Norris for his 9 a.m. German class in Room 207.

O'Dell, then a sophomore, was sitting in the second row when the black-clad Cho burst into the room and began firing. He dove under his desk but was shot through his upper right arm.

When Cho moved on, O'Dell and other students ran to the door (none of the classroom doors in Norris had locks) and closed it. Cho returned, but O'Dell and his classmates wedged their feet against the door and prevented him from re-entering.

Now in the third year of a four-year veterinary doctoral program, the 24-year-old Roanoke native was in his apartment about 2 miles off campus Thursday, studying for an ophthalmology exam, when he received a "VT Alert." There had been false alarms before, and he was hoping this was one of them.

Then he turned on the local news and realized this was the real thing.

"I never thought that I would see a school shooting at Virginia Tech while I was still a student here," he said.

Four and a half years ago, sociology professor James Hawdon was watching out the window of his sixth-floor office in McBride Hall as ambulances and SWAT vehicles swarmed the Drillfield. He stared in disbelief as officers carried bleeding students and colleagues from Norris, just 100 yards away.

On Thursday, he was again on that sixth floor, though in a different room. He and about 30 members of his department were going over strategies for next semester when the time and date reading on the wall-mounted digital monitor - like that now in nearly every other classroom and office on campus - flashed news about the shooting.

After a few moments of "I can't believe this is happening again," Hawdon said, people got up and went back to their respective offices to work. The only reason Hawdon didn't do the same is that his office is in the Center for Peace Studies & Violence Prevention - which occupies the remodeled Norris rooms where most of the 2007 victims perished.

"In terms of my personal safety, I did not feel particularly threatened on either incident, to be honest with you," said Hawdon, who took over as the center's chief when founding director Nowak retired this summer.