Brian Brennan, the commanding officer of the Nova Scotia Royal Canadian Mounted Police, said officers did not believe the attack was terrorist-related, but that the suspects had access to firearms.
Timberlea is a small town on the outskirts of Halifax in the remote Atlantic province of Nova Scotia.
The female suspect, from Geneva, Illinois, was arrested at Halifax airport after getting off a flight from Chicago and confessed to the plot.
An official said the woman had planned to meet a 20-year-old man at the airport before the killings.
He was also arrested, with a 17-year-old man arrested later yesterday.
Brennan said he believed police had detained all suspects involved.
"It's a group of individuals that were of the same ... mind to commit a heinous event and then take their own lives."
He said the plot was "not culturally based", adding: "Had [the suspects] been able to carry out their intentions the possibility for a large loss of life was definitely there."
Police warned one venue of the threat, but would not say where that was, said Canadian broadcaster CBC.