A woman from the US State of Illinois has admitted to plotting a massacre in a Canada mall on Valentine's Day. Lindsay Kantha Souvannarath unexpectedly pleaded guilty to conspiracy to commit mass murder during a routine court hearing, the BBC said.

Chris Hansen, a spokeswoman with the Nova Scotia Public Prosecution Service, said Souvannarath's plea on Tuesday (11 April) in Nova Scotia's Supreme Court "was unexpected". The 25-year-old was charged alongside her Canadian co-conspirator, Randall Steven Shepherd, for planning a massacre at the mall in Halifax, Nova Scotia, in 2015.

Souvannarath was expected to face trial in May, and her sentencing was scheduled for early October. Twenty-one-year-old Shepherd had already pleaded guilty in November 2016 and was handed a 10-year prison term.

The duo, along with a third person, James Gamble, had plotted online to commit mass murder using rifles and gas bombs at the Halifax Shopping Centre on 14 February, 2015, court documents reveal.

Souvannarath and Gamble met online in December 2014 and the 19-year-old started a relationship with her. Souvannarth is said to have "had a pre-existing interest in school shootings and Nazism".

The couple bonded over a mutual obsession with the 1999 Columbine High School attack in Colorado in which two teenagers killed 12 students and a teacher, before killing themselves.

They eventually started plotting the shooting throughout January and initial days of February 2015, targeting the shopping centre. They even codenamed their plan Der Untergang, which means "the downfall" in German.

The court filing said Gamble also kept a hesitant Shepherd informed of the plot.

However, the police foiled their plan a day before the intended shooting, following a tip-off just as Shepherd was picking Souvannarath from the Halifax airport. Gamble killed himself before the police could get to him.

The home in Timberlea, Nova Scotia, where the 19-year-old man was found dead. (Reuters)
The home in Timberlea, Nova Scotia, where Gamble was found dead Reuters