A woman accused of plotting a Valentine's Day attack at the Halifax Shopping Centre two years ago has entered a surprise guilty plea on a charge of conspiracy to commit murder.

Lindsay Kanitha Souvannarath pleaded guilty in the Supreme Court of Nova Scotia in Halifax on Tuesday. She had been scheduled to go to trial later this year.

Randall Steven Shepherd, her co-accused, pleaded guilty to the same charge last year and was sentenced in November to 10 years in prison, less 974 days for time served.

A third suspect, 19-year-old James Gamble, was found dead in his parents' home as police closed in.

Shepherd was arrested Feb. 13, 2015 at the Halifax Stanfield International Airport where he had gone to pick up Souvannarath, who was Gamble's online girlfriend. She had travelled to Nova Scotia from her home in Illinois to take part in the plot.

Police were acting on an anonymous tip through Crime Stoppers about people posting massacre plans on social media.

According to an agreed statement of facts presented during Shepherd's case, Souvannarath and Gamble were the main plotters behind the plan to open fire in the food court at the Halifax Shopping Centre.

Gamble planned to use his father's shotgun and hunting rifle. When they were unable to obtain more weapons, Shepherd agreed to make Molotov cocktails to increase the number of deaths.

Facts of the case showed that while Shepherd helped plan the attack, he intended to kill himself before the act.