Two people were found dead Thursday at Virginia Tech, one of them a police officer who was shot during a traffic stop, and the whereabouts of the suspect are unknown, the school said.

Shortly after noon, a Virginia Tech police officer made a routine traffic stop in the Coliseum parking lot near McComas Hall, and the 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.

Several law enforcement agencies have responded, including the FBI.

Virginia State Police have been requested to take 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 school, on its website, said the status of the shooter is unknown. School officials called on the campus community to remain on lockdown.

“Suspect 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.

“There is an active campus alert in Blacksburg,” the website said. “Everyone should seek shelter or stay where you are. Blacksburg Transit service is suspended until the alert is lifted.”

Brian Walls, a school employee, told CNN he heard 10 to 15 different sirens go by, and when he looked outside he saw a large group of officers. “I ended up seeing an individual on the ground,” he said, and rescue workers appeared to be attempting resuscitation.

“Then the campus alarm alerts went off,” and he got back inside, he said.

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.