A police officer was among two people shot dead Thursday on the Virginia Tech campus, prompting an intense search and university-wide lockdown that lasted about four hours.

About 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.”

Authorities did not directly answer questions late Thursday afternoon as to whether this second person to be fatally shot — described on the school’s website simply as “an unknown male subject” — is the prime suspect in the police officer’s shooting. Virginia State Police Sgt. Bob Carpentieri said only that “investigators feel confident that they have located” the officer’s shooter.

“When the university lifted the alert, they consulted with several people and they felt that it was safe for students and faculty to go about their day,” Carpentieri said. “We like to think that things are safe.”

The incident began about 12:15 p.m., when a four-year veteran of the Virginia Tech police department pulled over someone during a routine traffic stop in the Cassell Coliseum parking lot near McComas Hall. Cassell Coliseum houses athletic facilities and McComas Hall houses exercise facilities.

Soon thereafter, police “dispatch was unable to make contact with the officer,” said Virginia Tech Police Department Deputy Chief Gene Deisinger. Deisinger was deputy chief of police at Iowa State University until accepting the position at Virginia Tech in 2009.

Then, before 12:30 p.m., police received their first call from a witness who said he’d seen the suspect — who Carpentieri said authorities “do not believe … was in the vehicle that was stopped” — approach the officer’s car and open fire.

A few minutes later, at 12:36 p.m., the first of six alerts was issued to the campus community about the shooting.

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, said he also saw first-responders trying to resuscitate someone in the same area, which is also near the university’s football stadium.

The alleged shooter had fled on foot toward a parking lot known as “the Cage,” school spokesman Mark Owczarski said. This suspect was “described as white male, gray sweat pants, gray hat w/ neon green brim, maroon hoodie and backpack,” Virginia Tech said in posts on its website and Twitter feed.

About a quarter-mile from the first shooting, in what’s described as the “I-Lot,” a law enforcement officer saw a suspicious man and tried to contact him, Deisinger said.

“By the time they turned around and located the subject, that subject was deceased,” the deputy chief said.

This slain man was not identified by name, nor was the fatally wounded police officer. Police also did not give a possible motive for the shootings.

“We do not want to jeopardize this case,” Carpentieri said, explaining why authorities were not releasing further details. “No additional victims or shooting reports have been given to the police department. We feel confident that the situation is under control at this time.”

The Virginia State Police are taking the lead in the investigation, Deisinger said. Federal agents are involved as well, including six agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives who responded to the scene, ATF spokesman Scot Thomasson said.

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

The episode rattled a campus where 33 were killed on April 16, 2007, when student Seung-Hui Cho went on a shooting rampage. That incident took place at West Ambler Johnston Hall, across the street from the Coliseum lot.

“In light of the turmoil and the trauma and the tragedy suffered by this campus by guns, I can only say that words don’t describe our feelings and they are most elusive at this point in time,” Virginia Tech President Charles Steger told reporters Thursday.

On Thursday, law enforcement and school officials said the alert system put in place in recent years worked well, both from technological and communications standpoints.

“What I observed and experienced was significant cooperation from everybody I encountered,” Deisinger said of what he saw, on campus, after the alerts went out. “I think the community received the notification and responded to that, as we asked them to.”

Kiersten Todt Coon, a risk management consultant who was worked with other universities on campus management, said it appears that accurate information was communicated quickly and methodically — in contrast to what happened four years ago.

That said, she said school administrators should now review whether that information got out to everyone who needed to get it.

“Did the messages work across the board?” Coon asked rhetorically.

One school administrator said he and other staffers locked rooms and laboratories in Shanks Hall within three minutes of getting the first alert around 12:36 p.m. In the subsequent hours, he said faculty and employees got “regular and updated information” that he described as “very helpful” and calming.

“We’ve practiced this over time,” said Robert E. Denton Jr., head of Virginia Tech’s communication department. “This is kind of scary to go through … It brings back so much.”

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 to Saturday.

Besides the Virginia Tech campus, Thursday’s incident had ripple effects elsewhere in western Virginia.

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 Highway 81 was shut down Thursday afternoon, Virginia Department of Transportation spokesman Jason Bond said. The stop, on the highway’s northbound side, is about 9 miles from the Virginia Tech exit.

He did not know whether that move — which was ordered by state police — was related to the Blacksburg shooting. Carpentieri noted there was “some suspicious activity at that location,” though he also declined to say whether it was related to the Virginia Tech incident.

Several members of Congress 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,” referring to the school’s mascot.

Gov. Bob McDonnell thanked law enforcement agencies involved in the case and commended Virginia Tech’s leadership — including its president, 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 the 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 on Virginia Tech under the Clery Act, a federal law related to campus safety.

In April, Virginia Attorney General Ken Cuccinelli called the 2007 case against Virginia Tech “Monday-morning quarterbacking at its very worst.”

As to Thursday’s incident, Carpentieri stressed that there were “two crime scenes” — where the two bodies were found — and that the probe is ongoing, with many questions still unanswered. At the same time, he voiced confidence that “things are safe for people to be out again.”

“These are large crime scenes and it takes several hours to put the puzzle together,” the state police sergeant said. “We still don’t have a lot of the puzzle pieces connected.”

— CNN’s Carol Cratty, Phil Gast, Vivian Kuo, Eric Fiegel and Sally Holland contributed to this report.