The American woman convicted of plotting a 2015 attack on a Halifax mall has lost her appeal to overturn her sentence.

Lindsay Souvannarath pleaded guilty to a charge of conspiracy to commit murder for her part in the plot, which was planned for Valentine's Day 2015. She was sentenced to life in prison and the trial judge ordered that she serve a minimum of 10 years in prison before she could apply for release.

Souvannarath appealed the sentence, saying it was flawed and manifestly harsh and excessive. In a decision released Wednesday, the Nova Scotia Court of Appeal dismissed her arguments and upheld the life sentence.

Souvannarath flew to Halifax from her home near Chicago, Ill., to join her online boyfriend, James Gamble, in a plot they had been formulating for weeks. But an anonymous tip to Crime Stoppers alerted police and as they closed in on Gamble's suburban Halifax home, he took his own life.

Souvannarath was arrested at Halifax's Stanfield International Airport when her answers to routine questions aroused the suspicions of a Canada Border Services Agent. Gamble's friend, Randall Shepherd, was also arrested at the airport. He had gone there to pick up Souvannarath.

Shepherd eventually pleaded guilty to conspiracy to commit murder and was sentenced to 10 years in prison. In her appeal, Souvannarath pointed to that sentence to argue that her sentence was unreasonably long.

"The appellant and Randall Shepherd simply cannot be described as 'similar offenders' who committed 'similar offences' in 'similar circumstances', Justice Anne Derrick wrote for the three-member appeal panel.

"The disparity in their sentences was justifiable."

Gamble and Souvannarath had planned to go to the food court at the mall, armed with a rifle, a shotgun and Molotov cocktails and kill as many people as possible. Shepherd refused to take part in the actual attack and only agreed to supply the parts necessary to make the firebombs.

Co-accused Randall Shepherd pleaded guilty to conspiracy to commit murder and was sentenced to 10 years in prison. (Andrew Vaughan/The Canadian Press)

The Appeal Court also rejected the suggestion from Souvannarath's lawyer that the trial judge had improperly compared her offences to acts of terrorism or that her apparent lack of remorse was used against her in imposing the sentence.

"It is impossible to know with complete certainty if the appellant and Gamble would have made it to the mall with the guns and Molotov cocktails," Derrick wrote..

"The sentencing judge was entitled to determine the gravity of the conspiracy. He was required to decide what sentence would protect the public. That made it necessary for him to assess the appellant's ongoing dangerousness. He did so. He was satisfied beyond a reasonable doubt by the evidence that had the plan not been interrupted, the appellant and Gamble would have carried it out."

Crown prosecutor Tim O'Leary told the court last month that Souvannarath's sentence was not excessively harsh, but rather at the high end of what's recommended.

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