EDMONTON -- Police believe a robbery that ended with three armoured car guards shot to death at the University of Alberta was an inside job and investigators have issued murder warrants for one of the crew.

Edmonton police Supt. Bob Hassel said officers were searching for Travis Baumgartner, 21, who faces three charges of first-degree murder and one of attempted murder.

"We now believe we have reasonable and probable grounds that this is the person who is responsible for this horrific and terrible crime," Hassel, the head of criminal operations, said Friday.

"We sincerely believe Baumgartner is armed. He's dangerous. And we're urging public to use extreme caution should you happen to encounter this person."

Baumgartner was believed to be driving a Ford F-150 truck with Alberta licence plate ZRE-724. Edmonton police, RCMP, Canada Border Services officials and U.S. Customs agents were all involved in the search.

Baumgartner was one of five G4S guards making a delivery to a bank ATM machine in a mall-residence complex on the university campus just after midnight Friday.

Moments later passers-by came upon the ghastly scene. Two guards were found dead by the bank machine with a third critically wounded. The other guard was found dead by a G4S minivan on the street. Baumgartner was nowhere to be found.

The dead were two men and a woman. The critically injured guard was a man.

A second G4S vehicle, an armoured car, was found hours later parked and idling on the side of the road near an east-end G4S compound. Police haven't said how, or if, that car was involved.

The shootings occurred at HUB Mall, a long, thin rectangular block of shops, eateries and student apartments on the east end of campus.

One bystander photo of the scene posted to Facebook showed three people from G4S lying in front of the bank machine, emergency crews working over the bodies. There were blood streaks on the floor out from behind the machine to where the bodies were lying.

Ian Breitzke said he saw police pulling out bodies. The 21-year-old accounting student said he was watching TV in his residence room and heard a man in a room behind an ATM crying out in pain.

"When the police came in about 10 minutes, they ended up busting down the door (of the ATM room) and pulling out all the bodies that were in there," he said.

"Another couple of moments after that (they) pulled the man who was still alive out of the room."

The search for Baumgartner included the area around his home in Sherwood Park, on Edmonton's eastern outskirts.

Steven Munz, a friend of Baumgartner's, said his buddy had been with G4S for about three months and hoped eventually to become a police officer.

"But he felt he really didn't have what it took," Munz said.

He said Baumgartner completed two weeks of training in Calgary before starting the night shift in Edmonton.

"He said he enjoyed it more than any other job."

The pair had been friends in high school. He said Baumgartner like to play video games and had worked in the oilpatch and in construction before G4S.

He said he was surprised police think Baumgartner could be involved, but had noticed his personality change in the last year.

"Over the last year I've kind of noticed him slowly changing as a person. It's almost more irrational the way he thinks.

"I didn't think he was capable of something like this, but who knows, right?"

In a profile on the dating website Plenty of Fish, Baumgartner bills himself as an armoured car guard who is into gaming and recreational drug use.

There is a photo of him shirtless in a bathroom holding a cellphone.

He says he's six-foot-four with a laid back personality, a healthy workout regime and a "10" physique.

"I'm a great guy. We don't come around often," he writes.

G4S is an international security company with more than 630,000 employees. It has a specialized cash-management arm that delivers pay packets to fill ATMs. Steinberg said the Edmonton office, which has about 100 employees, has limited its operations since the shootings.

Executives from Toronto were flying to Edmonton to meet with staff and police.

"It's devastating," said G4S spokeswoman Robin Steinberg, who confirmed the deaths and injuries. Names were not released. "Our hearts go out to families of the victims."

Steinberg confirmed the guards were armed, but would say little else.

The university was put in lockdown after the shooting.

Grief counsellors were made available for students living at the HUB residence, and students too traumatized to write exams were being allowed to defer them.

About 560 students are living in residence at the mall at this time of year

Curt Binns, executive director of the ATM Industry Association Canada, said the amount of money put into bank machines at any one time varies widely.

"I worked at a branch of a bank right downtown and on weekends, when there would be things like the Carribana Festival or the gay (pride) parade, and a baseball game at the same time, we would put about $110,000 in the machine," he said.

"That's downtown Toronto where you have millions of foot traffic on a particular weekend. But if you're in (a town), population 500, and there's nothing happening on the weekend, the operator might put $1,000 in the machine."

It was the second robbery of a G4S armoured vehicle in Edmonton in recent months. Last December, guards making a mid-afternoon pickup outside a casino were attacked and pepper-sprayed by two masked men. The pair fled in a Jeep with an undisclosed amount of money. No arrests have been made.

In Calgary, two men dressed in military fatigues and armed with assault rifles and tear gas ambushed two Brinks guards delivering $300,000 in cash to a north-end mall in 1998.

More than 80 bullets were exchanged in a fierce gun battle, but no one was hurt. The robbers fled empty-handed and were later arrested.