Canadian police said they had foiled a plot by two suspects who were planning on going to a mall and killing as many people as they could before committing suicide on Valentine's Day in Halifax.
The suspects were on a chat stream and were apparently obsessed with death and had many photos of mass killings, a police official said on Friday.
The official said one of the two suspects, a 23-year-old American woman from Geneva, Illinois, was arrested at Halifax's airport and confessed to the plot.
She allegedly prewrote a number of pronouncements to be tweeted after her death.
Police said the suspects had access to firearms, but did not elaborate.
The official said the 19-year-old male shot himself to death after police were tipped off about the plot and they surrounded his home.
Police saw two people leave the house who they determined were his parents and pulled them over on a traffic check.
They then called the suspect. As the man told police that he didn't have any guns and he was on his way out of the house he shot himself, the official said.
The official spoke anonymously because they were not authorised to speak publicly.
Police earlier said two other Nova Scotia men, ages 20 and 17, were also involved, although investigators are still trying to determine what their roles were.
The police official said the 17-year-old male was wanted for threatening to shoot up a high school and had an outstanding warrant.
"Had they been able to carry out their intentions the possibility for a large loss of life was definitely there," Nova Scotia Royal Canadian Mounted Police Commanding Officer Brian Brennan said earlier on Friday.
"We believe we have apprehended all known individuals in this matter and have eliminated the threat. We are not seeking any further suspects at this time in relation to this investigation."
Canadian Public Safety Minister Steven Blaney applauded the work of police in Canada and Geneva, Illinois and well as border officials in a statement.
The Geneva police department said in a statement they were contacted by Canadian police and had no contact with the American woman before her arrest.
Canadian Justice Minister Peter MacKay has scheduled a news conference for Saturday morning.