MONTREAL (AFP) - Canadian police have charged two people in an alleged Valentine's Day plot to carry out a mass shooting at a Halifax mall then commit suicide, authorities said Saturday.

Lindsay Souvannarath, a 23-year-old woman from the US city of Geneva, Illinois was charged with conspiracy to commit murder along with Randall Shepherd, 20, from Halifax in Nova Scotia, Royal Canadian Mounted Police Assistant Commissioner Brian Brennan told a news conference.

The pair "had access to firearms and it was their intention to go to a public venue in the Halifax region on February 14th with a goal of opening fire to kill citizens, and then themselves," the RCMP said in a statement.

Another person thought to have been involved, a 19-year-old man, was found dead in a house in the rural town of Timberlea.

A fourth suspect, a 17-year-old boy from nearby Cole Harbor, was released without charge.

After receiving a tip about the alleged plot on Thursday, police said they acted quickly to track down the suspects.

Halifax police chief Jean-Michel Blais said the plot would have seen Souvannarath and the man who was found dead fire on crowds at the Halifax shopping centre, the Atlantic coastal city's largest mall.

They had then planned to commit suicide.

Justice Minister Peter MacKay said the suspects "appeared to be a group of murderous misfits ... prepared to wreak havoc and mayhem on our community."

Authorities say Shepherd was not going to participate directly in the killing but was allegedly helping in the plot.

He was arrested without incident along with the American woman at the Halifax Stanfield International Airport.

"This is a reminder that this type of incident can happen anywhere," Blais said.

"Recognising our citizens may be anxious in the wake of this news, we have additional high visibility patrols in and around the city. We encourage people to go about their daily lives."

The two suspects are due to appear in court Tuesday.

The region's Serious Incident Response Team was investigating the death of the man in Timberlea, a small town on the outskirts of Halifax in the rugged and remote Atlantic province of Nova Scotia.

Steven Blaney, the minister for public safety, on Friday praised the police operation to thwart the attack.

"These arrests are a great example of the fine work they do on a daily basis to help keep Canadians safe," he said in remarks reported by the CBC.

"We support our law enforcement agencies who work tirelessly to ensure our communities are safe places to live, work and raise families," he said.