Virginia Tech was in shock today after a school police officer and another person were shot dead in a chaotic scene on the campus where the deadliest school shooting in US history unfolded in 2007.

Police refused to confirm reports that the second fatality might have been the shooter himself, as the 31,000-student university lifted an afternoon-long lockdown and declared there was no longer an "active threat" late Thursday.

But when asked if the gunman was at large, Virginia State Police spokesman Bob Carpentieri cryptically told reporters: "Investigators feel confident that they have located the person."

Slain police officer Deriek Crouse, 39, a four-year veteran of Virginia Tech's own security force, was shot and killed during a noon-hour "routine traffic stop" in a parking lot near a sports facility.

"Once again, the campus and the community that we love so well have been visited by senseless violence and tragic loss," university president Charles Steger said in a statement.

"Tragedy again struck Virginia Tech in a wanton act of violence where our police officer, Deriek Crouse, was murdered during a routine traffic stop."

The university had earlier cited witnesses as saying that the shooter fled on foot heading toward the Cage, a parking lot near Duck Pond Drive.

"At that parking lot, a second person was found. That person is also deceased," the university said in a statement.

Carpentieri said a weapon was found close to the second victim, whom he described as a white male. He also said the gunman was not the motorist Crouse had pulled over.

Virginia Tech was the scene of the deadliest shooting incident by a single gunman in US history in April 2007, when an English major, Seung-Hui Cho, 23, killed 32 people before taking his own life. Twenty-five others were wounded.

The university -- criticized for what was seen as a slow response to the 2007 bloodbath -- put lessons learned into practice, issuing lockdown alerts by email, web and text message shortly after gun shots were reported.

Ironically, Virginia Tech officials -- including the campus police chief -- were in Washington on Thursday to appeal a $55,000 fine for failing to promptly issue an email alert after Cho shot his first two victims in a dormitory.

Thursday was a study day on the eve of final exams for the fall term, which have been pushed back a day until Saturday. Faculty have been asked to "make accommodations where appropriate."

One unidentified student, fighting back tears, told local WDBJ television she witnessed the fatally wounded police officer fall out of his unmarked vehicle when colleagues opened its door.

"He just fell out towards the ground, and then they immediately started reviving him, and then two cops took off with some sort of automatic weapons, I guess, running in the opposite direction after the gunman," she said.

"I guess the officer didn't make it because they just covered him with a sheet."

The gunman was described as a white male in a maroon hooded sweatshirt and grey sweatpants. Maroon is the official color of Virginia Tech.

Police with handguns drawn and assault rifles at the ready swarmed across the campus as students and staff took refuge for four hours in their dormitories, offices and a leisure center.

Virginia Tech employee Brian Walls told CNN he initially heard 10 to 15 different sirens wail and then went outdoors, where he found a large group of police officers.

"I ended up seeing an individual on the ground" and rescue workers apparently trying to revive a fallen person, he said. "Then the campus alarm alerts went off," and Walls went back inside.

Tauhid Chappell said he and other fellow students were getting information through the university's multitiered alert system as well as the Internet and radio, telling CNN the atmosphere on campus felt "very quiet and still."

"I was in my dorm all day and I saw just a bunch of action on campus," first-year student Andrew McElvarr told WDBJ television.

"Everyone (in the dormitory) is just waiting, watching, trying to figure out what's happening."

Virginia Governor Bob McDonnell said the university had "great resolve."

"I have no doubt that the students, alumni and faculty of this proud institution will emerge from this sad day stronger and more united than ever before."