Two people have died in a shooting at Virginia Tech university, including a campus police army veteran who was a father of five.
Officials said the gunfire began when Officer Deriek Crouse, 39, made a routine traffic stop on campus.
Multiple reports suggested that the gunman was the second victim, but there was no confirmation of this.
Virginia Tech was the site of the worst US school shooting, in 2007, when a gunman killed 32 people and himself.
"Our hearts are broken again," said university President Charles W Steger at an afternoon news conference.
On Thursday evening about 150 students gathered in a candlelight vigil at the campus memorial for the 2007 shootings. An official event is to be held Friday.
At the memorial, Reuters reported, a student left a note on orange paper. "We are the Hokies," it said, referring to the name of the school's sports teams. "We will prevail".
State police officer Sgt Robert Carpentieri told reporters it appeared that the shooter was not in the car that was pulled over.
The suspect fled towards another parking lot, where police found the second victim.
"Since the time of the second incidence there have been no other founded reports of any threat to the campus," Gene Deisinger, Virginia Tech's deputy chief of campus security, said.
Crouse had served on the campus police force for four years, joining the Virginia Tech Police Department about six months after 2007 shootings.
As events unfolded on Thursday police swarmed the campus looking for the shooting suspect, a white male, after he escaped on foot.
The campus was placed on "lockdown", with staff and students advised to remain in place and not go out while the alert was valid.
No classes were in session, as it was the day before the university's final exams. Those exams have now been postponed.
Virginia Tech said on its website: "Shortly after noon today, a Virginia Tech police officer stopped a vehicle on campus during a routine traffic stop in the Coliseum parking lot near McComas Hall.
"During the traffic stop, the officer was shot and killed. There were witnesses to this shooting.
"Witnesses reported to police 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's website said a weapon had been recovered "at the location of the second individual".
Officials lifted the lockdown at 1630 EST, and gave a news conference in which they confirmed the alert was over.
False alarm fears
The suspect was said to be wearing grey tracksuit bottoms, a grey hat with neon green brim, maroon hoodie and backpack.
Arsalan Heydarian, a graduate student, told BBC that he was in an engineering lab when the events happened.
"We heard sirens around the campus," he said. "We then started to get text messages via the new security system, telling students to stay where they are and stay away from windows."
Another student, Pranav Angara, said police cars surrounded his building and were going through all the dorms.
"We've had false alarms like this and I thought it might be another false alarm," he said.
The incident came on the same day as Virginia Tech appealed against a $55,000 (£35,200) fine imposed by the US Education Department for not reacting quickly enough to the April 2007 massacre.
Mr Heydarian was a freshman at Virginia Tech in 2007.
"Now I'm graduating with my masters degree in a few months, and have witnessed a second round of shootings," he said.
Thirty-two people died in 2007 when a 23-year-old South Korean, Seung-Hui Cho, went on a gun rampage before turning the weapon on himself.
The university, which has about 30,000 students, implemented a highly advanced security alert system after the tragedy.
It was put to the test in 2008, when an exploding nail gun cartridge was mistaken for gunfire.
On Thursday, the university issued several alerts as the situation developed across campus.