Image copyright CBC Image caption Lindsay Kantha Souvannarath was obsessed by school shootings and Nazism

A US woman has admitted plotting to carry out a mass shooting in a Canadian shopping centre on Valentine's Day.

Lindsay Kantha Souvannarath, 25, made the surprise plea to conspiracy to commit murder, during a routine court hearing.

She was charged alongside Canadian Randall Steven Shepherd for planning a massacre at the mall in Halifax, Nova Scotia, in 2015.

They wanted to use rifles and gas bombs to kill shoppers at a food court.

Shepherd pleaded guilty in November and was sentenced to 10 years in prison.

Chris Hansen, a spokeswoman with the Nova Scotia Public Prosecution Service, said the American's plea on Tuesday in Nova Scotia's Supreme Court "was unexpected".

Image copyright Facebook Image caption Randall Steven Shepherd pleaded guilty in November and was sentenced to 10 years in jail

Souvannarath, of Geneva, Illinois, was scheduled to go on trial in May.

According to an Agreed Statement of Facts filed in the province's top court, 21-year-old Shepherd, Souvannarath, and a third person, 19-year-old James Gamble, plotted online to commit mass murder on 14 February 2015 in Halifax.

In December 2014, Gamble began an online relationship with Souvannarath, who "had a pre-existing interest in school shootings and Nazism".

They bonded over a shared obsession with the 1999 Columbine High School attack in Colorado and other mass shootings.

Image copyright Not specified Image caption James Gamble reportedly made sinister references to mass shootings on a blog

Gamble eventually confessed his desire to commit mass murder, and the two began plotting the shooting spree throughout January and early February of 2015, targeting the Halifax Shopping Centre.

They even gave their plot a codename, Der Untergang, which means "the downfall" in German.

Gamble kept a hesitant Shepherd abreast of the plot, according to the court filing.

Image copyright CBC Image caption Lindsay Kantha Souvannarath in court

Police discovered the plans following a tip-off from Crime Stoppers just a day before the shooting spree was due to take place, as Shepherd was collecting Souvannarath from Halifax airport.

Gamble committed suicide as police closed in.

Souvannarath's sentencing is scheduled for early October. The Crown has yet to make sentencing recommendations.