Randall Shepherd, 21, described by the defense as a disenfranchised youth, pleaded guilty to conspiracy to commit murder.
"I'm deeply sorry this has happened," he reportedly told the Nova Scotia court.
A spokesperson for the Crown told AFP the prosecution and defense jointly recommended to the judge a 10-year sentence.
American alleged co-conspirator Lindsay Souvannarath, 24, meanwhile, still faces prosecution in the plot. Her case is scheduled to go to trial in May.
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Acting on a tip, police arrested Shepherd and Souvannarath at the Halifax airport and foiled the plot. Shepherd was picking up Souvannarath, who had arrived on a flight from her hometown of Geneva, Illinois.
Shepherd and Gamble had met in high school. They bonded over "heavy metal music, gore/horror movies, marijuana and a shared fascination with death and morbidity, including school shootings and mass murders," said an agreed statement of facts.
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