Moments before four young people were shot dead in Alberta this week, one car rear-ended another to stop it on a highway near Lethbridge, Calgary RCMP said on Saturday.

The Mounties said the car that rammed the other was being driven by Derek Jensen, 21, who had been looking for his ex-girlfriend early Thursday after a confrontation at a pub.

He was following her car after spotting it at a 7-Eleven convenience store in Claresholm. Jensen had three loaded and registered guns with him -- a handgun, a shotgun and a rifle.

They discovered another rifle and ammunition at his home in Lethbridge. Police confirmed that the guns were registered.

When the driver, Shayna Conway, 21, got out of the damaged car, she was shot several times with a handgun.

Conway remains in hospital recovering from multiple gunshot wounds and is expected to recover. Police said her account of the events was instrumental in piecing together the incident.

The RCMP said Jensen then approached the car and fired several shots, killing his ex-girlfriend Tabitha Stepple, 21, and Tanner Craswell, 22. Mitchell MacLean, 20, who was also shot, escaped from the car and crawled into a ditch. He died on the way to hospital.

Jensen then shot himself.

RCMP Sgt. Patrick Webb said investigators are examining "a domestic violence, jilted-boyfriend motive" in the slayings.

"We can surmise there was definitely a certain amount of planning in this," Webb told a news conference in nearby High River. "No one drives around for the most part with three loaded weapons. Exactly how they were to be utilized or what his intentions were, we may never know."

Witnesses told CTV News that Stepple's former boyfriend Jensen went "berserk" when he found Stepple and the other three celebrating Craswell's birthday in a Lethbridge pub on Wednesday night.

"He saw her and lost it, lost his mind ... and he was like yelling," said Caitlin McFarland, a friend of Stepple's. "Then we left, he was phoning her, phoning her, phoning her and said to her, 'This night's not going to end well for you. I hope you know that.'"

McFarland said Jensen pushed Stepple out of her chair during the confrontation at the pub. The couple broke up a few months earlier but were still living together, although Jensen was supposed to have moved out the day of the shooting. About the breakup, Webb of the RCMP said, "Jensen was not satisfied with that."

Early Thursday morning, the bodies were discovered on the side of Highway 2 north of Lethbridge. Friends said Stepple and Conway were taking the two men to the Calgary airport for a flight back home to Prince Edward Island for Christmas.

According to the couple's friends, Jensen, an avid hunter and skeet shooter who had recently completed training as a paramedic, was a nice boy from a good Mormon family but a possessive and jealous boyfriend.

"She would tell me he's very controlling," said Claire Sullivan, a friend of Stepple's. "She would say things like, 'I'm not allowed to do things without him. He wants to know where I am and what I'm doing.'"

MacLean and Craswell were baseball players and best friends who played together on the Lethbridge Bulls, a collegiate summer baseball league. MacLean was named rookie of the year and Craswell was an all-star shortstop.

"The whole P.E.I. community is saddened," said Baseball P.E.I. president Don Leclair, who had watched the young men's short careers. "A number of my children went to school in western Canada and I know the excitement you felt waiting at the airport and now these people are waiting for their caskets to come."

In Edmonton, Jimmy Morrison of P.E.I. mourned the loss of two friends.

"They weren't friends, they were brothers," he said. "Two great talents, you know?"

The coach of the Lethbridge Bulls said he had planned to drive his two players to the airport the night before but they had decided to stay an extra day to celebrate Craswell's birthday. Then Stepple offered to drive them to the airport after the party.

"Those boys had nothing to do with (Jensen)," said Kevin Kwane, who was also the landlord for the players. "They were in the wrong place at the wrong time."

Students at the former high school of two men from P.E.I. plan to hold a game of catch in their honour next week.

When classes end Monday, Colonel Gray High School in Charlottetown will hold the game to honour MacLean, of Cornwall and Craswell, of Charlottetown, the school's principal Kevin Whitrow said Saturday.

Whitrow, who is organizing the event, said he expects hundreds of people to attend the unique event.

"That's what we're expecting that there will be a whole lot of people who bring a ball and a glove and kind of just play catch and remember... the guys."

Participants will have the opportunity to sign a guest book, said Whitrow.

He said it's been an emotional few days at the high school and in the community.

"These were boys that even if you didn't teach them, you knew them," said Whitrow. "Everybody is caught by the sheer tragedy of this."

With files from The Canadian Press and CTV Calgary's Kaella Carr, Bridget Brown