HALIFAX—An American woman has pleaded guilty in a plot to kill shoppers at a Halifax mall on Valentine’s Day, a potential massacre avoided by a Crime Stoppers tip to police.

Lindsay Kantha Souvannarath of Geneva, Illinois, was one of three people involved in the 2015 plan to use rifles and gas bombs on people at the Halifax Shopping Centre’s food court.

Chris Hansen, a spokeswoman for the public prosecution service, says the 25-year-old woman caught prosecutors off guard when she pleaded guilty to conspiracy to commit murder Tuesday during a routine appearance before the Nova Scotia Supreme Court.

“This removes the burden of conducting a lengthy trial ... I can say it was unexpected,” said Hansen, adding sentencing was scheduled for Oct. 5 and 6.

One of the trio of plotters, Randall Steven Shepherd, 22, pleaded guilty last year to conspiracy to commit murder and was sentenced to a decade in jail.

An agreed statement of facts in that case said that Souvannarath and James Gamble, a 19-year-old friend of Shepherd’s, had planned to attack the mall.

Gamble had started an online relationship with Souvannarath in December 2014, and they exchanged explicit images.

“Ms. Souvannarath had a pre-existing interest in school shootings and Nazism. The two quickly bonded over their shared interest in Columbine and other mass shootings,” it said, adding that Gamble revealed his desire to commit a mass shooting to Souvannarath.

He said Shepherd, his only friend, wasn’t overly interested in a mass murder and asked her if she’d take his place.

“Souvannarath was interested in being his partner and was eager to participate. They planned their attack in great detail throughout January and early February of 2015,” said the statement, noting they planned details such as the time, place, weapons to be used and “whether they would taunt the victims.”

The statement of facts says the location was selected because they believed it would cause the maximum amount of panic.

Gamble had obtained his father’s firearms — a lever-action hunting rifle and a single-action, 16 gauge shotgun — which he and Souvannarath planned to use.

“Both carefully selected ‘death outfits’ to be worn during the shooting, which included wearing masks ... A musical soundtrack was created and they agreed to post a video of the shooting on the Internet to document the massacre.”

The pair named their plan “Der Untergan,” which is German for “the downfall,” and the massacre was to end with their own suicides.

During a preliminary hearing last July, court heard that the crime was foiled thanks to a Crime Stoppers tip that indicated a threat to a group or individual.

A detective tracked down Gamble at his home in nearby Timberlea on Feb. 13, 2015. Instead of having uniformed officers approach the residence to make an arrest, investigators decided it would be best, for safety reasons, to talk to the teen over the phone.

When Gamble was reached by phone around 9 p.m., an officer said he informed the teen that he would be arrested and taken to the police station for questioning regarding alleged threats made through social media.

As their five-minute conversation concluded, the officer said he heard a gunshot, then some clicking sounds on the phone.

It took an emergency response team three hours to evacuate nearby homes before they entered Gamble’s house and found his lifeless body.

Officers sent online photographs of Souvannarath to the Canada Border Services Agency at the Halifax airport, instructing agents there to detain anyone matching her description arriving on a flight that night from Chicago, via New York.

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When Souvannarath arrived, she was detained by border agents. Police said she had very little in her luggage, except a book on serial killers and her “death suit.”

Halifax regional police Const. Robert Fox testified Souvannarath told him the plan was for her and her “boyfriend” to kill themselves in a mall or a library.

“She said she had come to Canada to meet her boyfriend and they would kill themselves,” Fox told the hearing. “She said there were no other targets.”

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