The man who authorities say killed a Virginia Tech police officer was described as a typical college student in many ways, making it difficult to understand why he would commit an armed robbery and then, apparently at random, target the patrolman before killing himself.

The gunman was identified Friday as Ross Truett Ashley, a 22-year-old part-time business student at Radford University, about 10 miles from the Virginia Tech campus in Blacksburg. He first drew authorities' attention Wednesday when, they say, he walked into his landlord's office with a handgun and demanded the keys to a Mercedes-Benz SUV.

As investigators worked to unravel a motive, thousands of people gathered for a candlelight vigil Friday night on a campus all too familiar with tragedy.

Those who knew Ashley said he could be standoffish. He liked to run down the hallways and recently shaved his head, a neighbor said.

Virginia State Police said he walked up to officer Deriek W. Crouse after noon on Thursday and shot him to death as the patrolman sat in his unmarked cruiser during a traffic stop. Ashley was not involved in the stop and did not know the driver, who is cooperating with police, they said.

Authorities said Ashley then took off for the campus greenhouses, ditching his pullover, wool cap and backpack as police quickly sent out a campus-wide alert that a gunman was on the loose. Officials said the alert system put in place after the nation's worst mass slaying in recent memory worked well, but it nevertheless rattled a community still coping with the day a student gunman killed 32 people and then himself.

A deputy sheriff on patrol noticed a man acting suspiciously in a parking lot about a half-mile from the shooting. The deputy drove up and down the rows of the sprawling Cage parking lot and lost sight of the man for a moment, then found Ashley shot to death on the pavement, a handgun nearby. No one saw him take his life and he wasn't carrying any ID.

State police spokeswoman Corinne Geller said Ashley appears to have acted alone and didn't know the slain officer: ''At this time we have no connection between the two of them, that they knew one another or had encountered one another prior to the shooting,'' she said.

Ashley lived in an apartment on the top floor of a worn, gray three-story brick building in the small city of Radford, a college town. He lived above a yogurt shop, consignment store, barber shop and a tattoo parlor.

On Friday night, students popped in and out of the building visiting friends. Mandy Adams, a Radford grad student, said Ashley had recently shaved his head. Other than running down the hallways, he was quiet, she said.

''He would just run down the hallway -- never walk, always run,'' said Adams, who was out on a rear fire escape with a glass of white wine and a cigarette to calm her nerves. ''It's going to be really creepy when they come to take his stuff out of here.''

Neighbor Nan Forbes, a Radford senior, said Ashley was rarely seen or heard from. She said she knew he was in trouble when she saw two police officers guarding the door to his apartment

''It does freak us out because we live in this building, but there was not one peep of trouble, nothing unusual,'' she said.

Ashley made the dean's list in 2008 at the University of Virginia-Wise, which is located in southwest Virginia. He took classes at Radford, a former state teachers college in the Blue Ridge Mountains that now has more than 9,000 students.

Officials at Radford or UVA-Wise were not immediately able to talk in detail about Ashley.

At the Virginia Tech campus, thousands of people silently filled the Drillfield for a candlelight vigil Friday night to remember Crouse, a firearms and defense instructor with a specialty in crisis intervention. He had been on the campus force for four years, joining it about six months after the April 16, 2007 massacre.

Crouse was a member of the Army Reserves who served a year in Iraq beginning in March 2004, according to the U.S. Army Human Resources Command. He was assigned to active duty service at Fort Hood, Texas from October 1993 until July 1996, where he was listed as an M1 armor crewman, or tank operator. From July 1996 to May 2001, Crouse was listed as a motor transport operator with the 316th Sustainment Command in Galax, Va. Crouse's last rank was staff sergeant.

For about nine months in 2007, Crouse worked as an officer with the Montgomery County Sheriff's Office at the county's jail before leaving for the Virginia Tech police, said Capt. Brian Wright, a spokesman with the department.

Those who worked with Crouse remembered him as a ''great employee'' and a ''hard worker,'' said Wright, who had worked security with Crouse at Virginia Tech football games.

''He was just very personable, easy to talk to,'' Wright said. ''Everybody liked him.''

The Friday night vigil included a moment of silence and closed with two trumpeters stationed across the field from each other playing ''Echo Taps'' as students raised their candles.

''Let's go!'' one student then shouted.

''Hokies!'' everyone else responded.

Kathleen O'Dwyer, a fifth-year engineering major at Tech, said it was important to come for Crouse's family. Crouse was married and had five children and stepchildren.

''Also it's for the community, to see the violence that happens isn't what we're about,'' said O'Dwyer, who will be graduating next week.

Her plans when she leaves school?

''First, go home and hug my mom,'' O'Dwyer said.

Nobody answered the door Friday evening at Ashley's parents' home in Spotsylvania County, along the Interstate 95 corridor between Richmond and Washington. The house was dark and no vehicles were in the driveway. The two-story, log cabin-style home in a semi-rural area sits about 200 yards off the road up a narrow gravel drive.

Billie Jo Phillippe, who lives three houses down, said she didn't really associate with the family.

''They stay off to themselves a lot,'' she said. ''He was a clean-cut young guy but standoffish.''

Authorities declined to answer some questions about Ashley, including whether he had any mental health issues or was licensed to carry a handgun.

But Gov. Bob McDonnell commented briefly on the shooting while helping load presents into a van for the Marine Corps Reserves' Toys for Tots program.

''Some crimes, there's a relationship between a perpetrator and a victim, and some there aren't,'' said McDonnell, a former prosecutor and attorney general. ''There are random acts of violence, they involve either mental health issues, or robbery, or other motivations....Unfortunately in our society random acts of violence do occur, we unfortunately see it every day somewhere in this country.''

He said there's an ''extra degree of scrutiny'' of incidents at Virginia Tech because of the 2007 mass shooting.

''It's just unfortunate and almost inexplicable that you could have a series of these events happen in a short four-year period,'' the governor said.