EDMONTON -- When Baumgartner rolled up to the U.S. border, agents already knew who he was.

Thirty-six hours after being named a suspect, the 21-year-old man alleged to have gunned down four of his G4S security co-workers Friday at the University of Alberta was caught by U.S. border officials.

According to Edmonton Police Service Supt. Bob Hassel, who spoke to media at a midday news conference Sunday, Baumgartner "was arrested peacefully" Saturday around 3 p.m. by RCMP after being detained by U.S. border guards. Baumgartner was attempting to cross into the US at the Lynden, WA, US-Canada border, about 31 km south of Abbotsford, BC.

He was driving his dark blue Ford F-150 at the time of his arrest, with his mother's licence plate still attached.

No weapons were found in the vehicle, but Baumgartner did have a backpack with "just in excess of $330,000 Canadian currency," Chief U.S. Customs and Border Protection Officer Thomas Schreiber confirmed Saturday night.

Schreiber said the vehicle's plate was scanned on its way to the booth, where officers were waiting for him.

"We start making commands ... we don't ask, 'How's your day?' " he said.

"By the time a vehicle and its occupants arrive at the point of entry we have a pretty good idea who's inside."

Police said Baumgartner was believed to have a "significant amount" of cash which was allegedly taken as he and his co-workers stocked an ATM machine at the University's HUB Mall early Friday morning.

"We're grateful to the border officials at Lynden, WA, for their excellent work in arresting a man we believe was armed and extremely dangerous," Supt. Bob Hassel of the EPS' Criminal Investigations Division said.

The capture comes just hours after police issued a plea for friends who may have had contact with Baumgartner to come forward.

Baumgartner was one of five G4S security guards on duty who were exchanging funds at an ABM machine inside the University of Alberta's HUB Mall shortly after midnight.

At some point during the exchange, G4S guards Michelle Shegelski, Brian Ilesic and Eddie Rejano were shot and killed.

Matthew Schuman, another guard, was shot and remains in critical condition in hospital. Police have charged Baumgartner with three counts of first-degree murder and one count of attempted murder. No University of Alberta students inside the HUB Mall shopping centre and residence Friday morning were harmed or involved in the shooting.

A G4S armoured car, which police believe was the getaway vehicle used in the robbery, was discovered empty near a southeast Edmonton G4S facility early Friday.

Surveillance footage from the nearby Edmonton Sun press plant shows the driver of the armoured car park and retrieve something from the back of the vehicle before getting into a truck and leaving.

EPS commended all the friends, family and G4S employees involved for their co-operation.

-- With files from Damien Wood