The Nova Scotia Royal Canadian Mounted Police identified the woman as Lindsay Kantha Souvannarath, of Geneva, Illinois. She was arrested at Halifax Stanfield International Airport early Friday.
Souvannarath, 23, and Randall Steven Shepherd, 20, were charged with conspiracy to commit murder, police said.
"This is a reminder that this type of incident can happen anywhere," Halifax Regional Police Chief Jean-Michel Blais said.
A 19-year-old Canadian man who was believed to be linked to the plot died from a self-inflicted gunshot wound after officers surrounded his home, police said.
Officials said authorities received a tip that Souvannarath and the 19-year-old Canadian "had access to firearms, and it was their intention to go to a public venue in the Halifax region on February 14 with the goal of opening fire to kill citizens and then themselves."
Police said that "evidence also supports" that Shepherd was involved in the plot, but they did not release further details.
"It would have been devastating," Canadian Minister of Justice Peter Mackay said Saturday.
Mackay did not say what public venue was targeted, but said extra security was in place at the site.
Mackay said Souvannarath and the 19-year-old Canadian were chatting online. An official told The Associated Press that the two apparently were obsessed with death and had many photos of mass killings.
A 17-year-old who was arrested Friday was later released by police.
Police in Geneva, Illinois, who are assisting with the investigation, said today they never had any contact with Souvannarath and there is "no indication at all that there was any involvement locally."
The Geneva police added that her family has been "extremely cooperative."
Souvannarath and Shepherd are scheduled to appear in Halifax Provincial Court on Tuesday.
ABC News' Jack Date, Emily Shapiro and The Associated Press contributed to this story.
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