When RCMP Staff Sgt. Lisa Stuart learned of a Crime Stoppers tip that two Nova Scotia men and an American woman were planning a massacre in Halifax, she felt like she'd entered an episode of the TV thriller 24.

The anonymous tipster gave police a local name — 19-year-old James Gamble — and a warning that "Lindsay S. from Chicago" was on a plane to Halifax to carry out the murders.

Speaking to the CBC's Information Morning host Portia Clark Tuesday, a day after the van attack in Toronto, Stuart said that was a chilling reminder of what could have happened in Halifax.

"We've always felt a sense that it couldn't happen in Canada, but now, unfortunately, it has. Had it not been for this person who came forward and gave us the Crime Stoppers tip, we would have been talking about a different type of situation," she said.

"You just can never underestimate what a person's motives are and how they can carry that out."

Last week, Lindsay Souvannarath, 26, an admirer of the killers in the Columbine High School massacre, was sentenced to life in prison for her role in a plot to shoot and stab people in the food court at the Halifax Shopping Centre on Valentine's Day in 2015.

Now that the case has drawn to a close in court, Stuart, a supervisor with the integrated RCMP and Halifax Regional Police unit that investigated, spoke in candid detail about how the attack was foiled — and of her fears of what might have happened if police had not acted in time.

Blog photos give more clues

Back on Thursday, Feb. 12, 2015, police worked swiftly to figure out if the Crime Stoppers tip was true.

"We didn't know which mall. We had a name for the female, but we didn't have a last name. We also knew that their intention was to shoot and stab individuals at the mall," Stuart said. "It made the hairs on the back of your neck stand up."

A crime analyst searched social media sites, including Gamble's blog. They soon found alarming content.

"One in particular was him dressed in fatigue-like wear holding a shotgun and a rifle. The background of it looked like it was a local address. It wasn't a stock photo," said Stuart, who headed the operation.

When police saw this photo of James Gamble on his blog, they realized how serious the threat was. (Tumblr)

She sent a surveillance team to watch Gamble's house and told the Canada Border Services Agency to look for a Lindsay S. coming from the U.S. to the Halifax airport. Police won't reveal details of the tipster, but the person did not know Gamble personally.

Surveillance officers watched Gamble's mother and father leave the house. Officers stopped the parents and took them to the Tantallon RCMP detachment. The father said he had guns in the house, but they were under lock and key, so he didn't think his son could access them.

Police phoned the teenager on his cell and asked him to come outside to speak to police.

"At that time, he was co-operative and indicated to us that he was going to come outside," Stuart said.

Parents shocked

But after three minutes, a loud crack ended the phone call.

"It was at that moment that we realized that what we were dealing with was quite significant and that he was committed to a cause, in order to take his life so quickly having just spoken to officers for a few minutes."

A tipster led police to this Timberlea house, where Gamble planned to kill his parents ahead of the massacre. (CBC)

Stuart went to talk to his parents.

"They were very co-operative," she said. "I met with them personally. Obviously it was a very emotional situation because they were learning from friends that there had been a shot fired from the residence."

Police sent a robot into the Gamble house. It couldn't climb stairs, but seeing the first floor was clear, officers entered and found Gamble dead upstairs.

Later that evening, Gamble's parents got a call from their son's friend, Randall Shepherd. They told him Gamble was dead.

"It was a very short conversation. Basically he said, 'I've got to go,'" Stuart said.

There was now another suspect.

Randall Steven Shepherd planned to have his friend Gamble kill him before the attacks. He ended up with a 10-year prison sentence. (Canadian Press)

He called back later asking for their address, saying Gamble had asked him a week earlier to pick up a female arriving from Chicago. Around the same time, police told the Gambles that their son intended to kill them first.

"As a parent, to hear that information still bothers me today," Stuart said. "Had I let them go back into that residence that evening on the 12th, there's no doubt in my mind he would have shot them as they arrived home."

CBSA fails to spread alert for 'Lindsay S.'

A police officer at the airport spotted Shepherd in the waiting area and arrested him. Meanwhile, CBSA failed to get the alert to all agents handling incoming flights from the U.S.

Lindsay Souvannarath flew to Halifax to commit mass murder, but instead got a life sentence in prison. (Canadian Press)

Dominic Mallette, Atlantic director of CBSA, said the agency investigated why the alert wasn't shared, but would not say what was learned.

He praised the agent who independently grew suspicious of Lindsay Souvannarath, who is from Geneva, Ill., and sent her for a second interview.

"She had very bad teeth and her complexion was very bad with sores on her face. This made me think she may be on drugs," the agent noted in the agreed statement of facts presented in court.

"It became very clear to the primary inspection officer that the story didn't match. There was some discrepancy. She didn't have the money she needed, the plane ticket was purchased at the last minute, she was coming to visit a boyfriend, but didn't really know where he lived," Mallette said.

CBSA agents continued to think Souvannarath was a drug agent, and she told them she planned to spend a "memorable" Valentine's Day weekend with her boyfriend. An official then read the national lookout and realized Souvannarath was "Lindsay S."

'Halifax would never have been the same'

Later, a slew of online messages would emerge that showed how Souvannarath and Gamble met online over their shared fascination with the 1999 Columbine massacre in Littleton, Colo. They detailed a plan to kill as many people as possible at the Halifax Shopping Centre and use Molotov cocktails as bombs.

When police were sure all of the would-be killers were in custody, they called an extraordinary press conference on the evening of Friday the 13th to tell the public about the thwarted attack.

Looking back, Stuart said it's still surreal. "Halifax never would've been the same, had it happened."

Shepherd, considered a "cheerleader" in the plot, pleaded guilty to conspiracy to commit murder and was sentenced to 10 years in prison.

Stuart felt relief and pleasant surprise when Souvannarath was sentenced last week to life in prison with no chance of parole for 10 years.

"[Justice Peter] Rosinski really sent a message to anybody that finds themselves sharing the same ideology of mass shootings and school shootings, that this type of activity and criminal intent is not going to be taken lightly in our country."

She said the massacre in Toronto showed just how bad it could have gotten in Halifax. "We always need to vigilant. Had it not been for the person who gave us the Crime Stoppers tip, we would have been talking about a different type of situation."