BLACKSBURG, Va. (AP) -- Investigators believe the gunman who killed a Virginia Tech policeman acted alone and that he changed clothes after fleeing the scene, then killed himself with his handgun when another officer spotted him, state police said Friday.

Police spokeswoman Corinne Geller said investigators have not found anything connecting the gunman and the slain officer, Deriek W. Crouse, who was shot in his car Thursday in a campus parking lot after pulling over a driver for a traffic stop. The motive remains a mystery, she said.

"That's very much the fundamental part of the investigation right now," Geller said at a news conference.

The gunman was not a student at Virginia Tech, the scene of the deadliest gun rampage in modern U.S. history in 2007. Geller said investigators were confidant they know the gunman's identity but she declined to say anything more about his name, age or hometown until the medical examiner confirms his identity and next of kin are notified.

The campus shooting prompted officials to lock down the university for hours while police and SWAT teams searched the school.

Authorities have in-car video from Crouse's cruiser that shows a man with a handgun at the officer's car at the time of the shooting.

Geller laid out the most detailed account thus far of the shooting. She said Crouse had pulled over a car driven by a student and was stopped on a campus parking lot with the car in front of his cruiser. She said the driver, who she didn't name, had no connection to the shooting and has been very helpful to investigators.

Crouse was sitting in his cruiser when the gunman walked up and shot him. Geller declined to say if the officer was wearing body armor or where exactly he was shot. He was not able to return fire, she said.

The gunman fled on foot and went to nearby greenhouses, where investigators say he changed out of a pullover wool cap and left them there with his backpack.

Geller said a deputy sheriff on patrol then noticed a man at the back of a parking lot about half a mile from the shooting. The man was by himself and acting "a little suspicious." The officer drove around to approach him, lost sight of the man and then found him on the ground. The man appeared to have a self-inflicted gunshot wound and a handgun was nearby.

The events unfolded on the same day Virginia Tech officials were in Washington, fighting a federal government fine over their handling of the 2007 massacre where 33 people were killed. The shooting brought back painful memories. About 150 students gathered silently for a candlelight vigil on a field facing the stone plaza memorial for the 2007 victims. An official vigil is planned for Friday night.

Crouse was an Army veteran and married father of five children and stepchildren who joined the campus police force in October 2007. He previously worked at a jail and for the Montgomery County sheriff's department.

Crouse was one of about 50 officers on the campus force, which also has 20 full- and part-time security guards. Crouse received an award in 2008 for his commitment to the department's drunken driving efforts. He was trained as a crisis intervention officer and as a general, firearms and defensive tactics instructor.

The university also said its counseling center would be open all day Friday for students.

"A lot of people, especially toward the beginning, were scared," said Jared Brumfield, a 19-year-old freshman from Culpeper, Va., who was locked in the Squires Student Center.

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Associated Press writers Eric Tucker, Kimberly Hefling and Ben Nuckols in Washington; Michael Felberbaum, Larry O'Dell, Steve Szkotak, Bob Lewis and Dena Potter in Richmond, Va.; and Brock Vergakis in Norfolk, Va., contributed to this report

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