A man and a woman planned a Valentine's Day massacre at a Canadian shopping mall in which they would gun down as many people as they could before killing themselves, police have said.

A police source said the suspects, who were on a chat stream, were apparently obsessed with murder and death and had many photos of mass killings.

One of them, a 23-year-old American woman from Geneva, Illinois, was arrested at Halifax's airport and confessed to the plot, according to the source, who said she pre-wrote a number of pronouncements to be tweeted after her death.

The other suspect, a 19-year-old man, killed himself after police surrounded his home in Timberlea, a suburb of Halifax.

Police saw two people who they determined were his parents leave the house and stopped them for a traffic check. They then called the suspect and as he told officers that he did not have any guns and was on his way out of the house, he shot himself, the source said.

Authorities have said the plot was not terror-related.

Nova Scotia Royal Canadian Mounted Police said it received a tip-off on Thursday morning that the two suspects had access to firearms and intended "to go to a public venue in the Halifax region on February 14 with a goal of opening fire to kill citizens, and then themselves", commanding officer Brian Brennan said.

Police said two other young men, aged 20 and 17, had been arrested in connection with the plot. The police source said the 17 year old was wanted for threatening to carry out a shooting rampage at 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," Mr Brennan said.

"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."

Public safety minister Steven Blaney praised the work of the police in Canada and Illinois, and well as border staff.

The Geneva police department said it was contacted by Canadian police and had no contact with the American woman before her arrest.