Souvannarath, a 23-year-old from Geneva, Ill., and Shepherd, 20, of Halifax, were charged Saturday.
Souvannarath was arrested at the Halifax Stanfield International Airport after arriving in Canada early Friday, at around 2 a.m. Shepherd, who was there to meet her, was also arrested.
Later on Friday, police tracked down another suspect, James Gamble, 19, who was found dead at the home of his parents on Tiger Maple Drive in Timberlea, about 20 minutes outside Halifax. His death is under investigation by Nova Scotia's Serious Incident Response Team.
Gamble left behind a blog filled with images of death, shootings and references to the 1999 Columbine High School massacre in Colorado, and references to Adolf Hitler and Nazis.
A 17-year-old boy from Cole Harbour, N.S., was arrested, but has since been released without charges. Police said there is no evidence to link him to the alleged shooting plot, but the investigation is ongoing.
'A group of murderous misfits'
"We have averted a true tragedy here in Halifax," Justice Minister Peter MacKay told reporters after the arrests. "It would have marked our city and province forever."
When asked if the suspects had "Columbine" beliefs, MacKay responded, "I cannot confirm that, but what I can tell you is that this appeared to be group of murderous misfits … prepared to wreak havoc and mayhem on our community."
Brian Brennan, Nova Scotia RCMP commanding officer, has said a Crime Stoppers tip helped foil the alleged plot, but wouldn't provide more information about where the tip came from.
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