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

Around 4:30 p.m., the school in Blacksburg, Virginia, 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."

Police said the incident started shortly after noon, when the police officer made a traffic stop in the Coliseum parking lot near McComas Hall. During that stop, a person who wasn't involved in the traffic stop approached and shot the officer, authorities said.

After witnesses told police that the shooter fled toward a nearby parking lot, an investigating officer saw a "suspicious person" alive in that lot. When the officer circled back to approach the person - described as a white male - that person was dead of a gunshot wound, police said.

A Virginia state police spokesman said that police didn't shoot the second person, and he wouldn't say whether the person was a suspect, or whether investigators still are looking for a shooter. But he said investigators feel "confident we located the person," and that "you can kind of read between the lines."

The incident comes more than four years after a shooting spree in which a Virginia Tech student, 23-year-old Cho Seung-Hui, shot dead 32 people on the campus before killing himself in April 2007.

The following are running updates on Thursday's incident:

[Updated 5:15 p.m. ET] The Virginia Tech police officer who was fatally shot during a traffic stop in a campus parking lot shortly after 12:15 p.m. was shot by someone who wasn't involved in the traffic stop, police said at a news conference. The shooter allegedly walked up to the officer's vehicle and opened fire, police said.

[Updated 5:11 p.m. ET] Describing the death of the second person, authorities at a news conference said a police officer investigating the slain officer's death saw a "suspicious person" alive in a campus parking lot, and when the officer circled back the approach that person - a white male - he was dead from a gunshot wound.

Police declined to say whether that person shot himself, but a Virginia State Police spokesman told reporters that police did not shoot him.

The state police spokesman wouldn't comment on whether the second deceased person had been the person who killed the police officer. He declined to say whether investigators still are looking for a shooter, but investigators feel "confident we located the person."

"You can kind of read between the lines," he said.

The school's website noted that a weapon had been discovered near the second person - described as an "unknown male subject who was found deceased in a parking lot near the Duck Pond."

The school has said there is no longer a threat on campus, and that people could resume normal activities on campus.

[Updated 4:37 p.m. ET] A weapon was found at the location of the body of a second person killed on the campus, Virginia Tech says. That person, a male, was found in a parking lot, the school said.

[Updated 4:34 p.m. ET] Police have determined "there is no longer an active threat or need to secure in place," Virginia Tech has said on its Twitter feed.

The posting told students and staff they could now "resume normal activities."

[Updated 3:48 p.m. ET] 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, U.S. Department of Education officials said, according to CNN's Sally Holland.

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.

In April, Virginia Attorney General Ken Cuccinelli called the case against Virginia Tech "Monday-morning quarterbacking at its very worst."

[Updated 3:45 p.m. ET] In a message on its website, 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."

[Updated 3:07 p.m. ET] Students' final exams that were scheduled for Friday have been postponed, Virginia Tech said on its website. Friday would have been the first day of final exams for the semester.

"A decision regarding the final exam schedule will be made as soon as we have more information," the message said.

[Updated 2:46 p.m. ET] Tauhid Chappell, a student on campus, told CNN that 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 said. Classes for the semester ended Wednesday. Students were preparing Thursday for final exams.

[Updated 2:45 p.m. ET] A university spokesman says police have a description of a suspect and are searching “absolutely everywhere for the person who matches the description."

The campus is on lockdown, and people on campus are urged to "shelter in place," the spokesman said. The university is urging that no one visit campus, he said.

[Updated 2:10 p.m. ET] Six agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives are at Virginia Tech to help with the response to Thursday's shooting, ATF spokesman Scot Thomasson said, according to CNN's Carol Cratty. Twenty additional agents are on standby, he added.

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

[Updated 2:07 p.m. ET] The staff of the Collegiate Times newspaper has been evacuated from its offices in the Squires Student Center on the Virginia Tech campus, the paper's Twitter feed reports.

[Updated 1:56 p.m. ET] A Virginia Tech police officer has been shot and killed after making a traffic stop on the university's campus, Mark Owczarski, the university's director of news and information, said Thursday.

The suspect fled on foot toward another parking lot where a second person was also found dead, he said.

The shooter is still at large, he said.

[Updated 1:39 p.m. ET] Sirens were heard around the Virginia Tech campus Thursday, and rescue workers were trying to resuscitate a person who was on the ground, school employee Brian Walls told CNN. The school was on lockdown after shots were reported, and the school's website said a police officer was shot.

Follow the incident via the Twitter feed of the school's newspaper, the Collegiate Times. These reports have not been confirmed by CNN.

[Updated 1:16 p.m. ET] A police officer has been shot on or near the campus of Virginia Tech, the university's website says. A possible second victim is reported down in a parking lot and the suspect is at large, the website says.

Classes for the semester ended Wednesday. Students were preparing Thursday for final exams.

[Posted 1:01 p.m. ET] The campus of Virginia Tech University in Blacksburg is on lockdown after a report of shots being fired on the campus.

The suspected shooter is described as a white male wearing gray sweat pants, a gray hat with a neon green brim and a maroon hoodie and carrying a backpack, according to the university's website.

Those on campus should seek shelter or stay where they are, the website says.

In April 2007, Cho Seung-Hui, a 23-year-old Virginia Tech student, shot dead 32 people on the campus before killing himself.

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