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A police officer was among two people killed Thursday on the Virginia Tech campus, prompting a sweeping search and school-wide lockdown that lasted over four hours.

Around 4:30 p.m., the school announced on its Twitter feed that “law enforcement agencies have determined there is no longer an active threat or need to secure in place. Resume normal activities.”

The school’s website noted that a weapon had been discovered near the second victim — described as an “unknown male subject who was found deceased in a parking lot near the Duck Pond.”

The incident began shortly after noon, when a Virginia Tech police officer made a routine traffic stop in the Coliseum parking lot near McComas Hall. That officer was shot and killed, school spokesman Mark Owczarski said. There were witnesses to the shooting, he said.

The shooter fled on foot toward a parking lot called “the Cage.” At that lot, a second person was found dead, Owczarski said.

“We have many law enforcement agencies here. They have a description of the suspect. They are searching all buildings, all public areas, outdoor, indoors, we are looking absolutely everywhere for the person that matches the description of the suspect,” he said around 2:30 p.m.

The university knocked down what it called “unfounded” reports of additional gunshots reported heard since the original shooting, saying “there are no new substantiated reports of suspicious or criminal activity.”

The Virginia State Police is taking the lead in the investigation, Owczarski said.

Six agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives responded to the scene, ATF spokesman Scot Thomasson said. Twenty additional agents are on standby, he added.

“The agents are very familiar with the campus and the buildings,” Thomasson said, as they have worked with Blacksburg police since the 2007 mass shooting on campus.

In April 2007, 33 people were killed when student Seung-Hui Cho went on a shooting rampage at Virginia Tech. That shooting took place at West Ambler Johnston Hall, across the street from the Coliseum lot.

The suspect in Thursday’s shooting was “described as white male, gray sweat pants, gray hat w/neon green brim, maroon hoodie and backpack. On foot towards McComas,” Virginia Tech said in posts on its website and Twitter feed.

All public schools in Montgomery County — where Blacksburg is located — were also placed on lockdown, county school superintendent Brenda Blackburn said. Dismissal eventually began at 3:30 p.m., after a delay of more than an hour.

In addition, a western Virginia rest area along Interstate 81 was shut down Thursday afternoon, Virginia Department of Transportation spokesman Jason Bond said. He did not know whether that move — which was ordered by state police — was related to the Blacksburg shooting. The stop, on the highway’s northbound side, is about 9 miles from the Virginia Tech exit.

As to what happened hours earlier, a visibly upset Julie Fleming described to CNN affiliate WDBJ how she’d been walking nearby and saw police pull up around the vehicle of the officer who had been shot.

“When they opened (the car door), he just fell out toward the ground and they immediately started reviving him,” she said, adding that two officers then took off with weapons after the gunman.

Brian Walls, an employee in the school’s athletic department, told CNN he also saw first-responders attempting to resuscitate someone in the same area, near the university’s football stadium and basketball arena.

Tauhid Chappell, a student on campus, said he and other students were getting information through the school alert system, and through the web and radio.

Things felt “very quiet and still,” he told CNN.

The Cassell Coliseum at Virginia Tech houses athletic facilities. McComas Hall houses exercise facilities.

Classes for the semester ended Wednesday. Students were preparing Thursday for final exams for the fall semester, though the university announced on its website that Friday’s exams had been postponed until further notice.

Several U.S. congressmen from Virginia issued statements in the wake of the shooting Thursday, including Rep. Morgan Griffith who asked people to “join me in lifting up the Hokie Nation in prayer.”

Gov. Bob McDonnell thanked law enforcement agencies involved in the case and commended Virginia Tech’s leadership — including its, President Charles Steger, whom he said he’d talked with Thursday — and its students “who cooperated to efficiently respond to this emergency.”

“Since this news first broke, we have been monitoring the situation closely and I have made available all requested state resources to Virginia Tech, including the Virginia State Police,” McDonnell said in a statement.

Several Virginia Tech administrators — including the head of campus police, Wendell Flinchum, and emergency management director Michael Mulhare — were in Washington Thursday at a federal court hearing on the 2007 shooting on the school’s campus, according to U.S. Department of Education officials.

Flinchum and the school were criticized after that incident for not notifying students quickly enough that there was a danger on campus. The hearing was an appeal of the Department of Education’s fine of $55,000 to Virginia Tech under the Clery Act, a federal law related to campus safety.