Finnish authorities say they have arrested a 21-year-old woman suspected of plotting a school massacre.

Police said Tuesday they interrupted the woman's "extensive and careful planning" of the attack when they arrested her last week.

Detective Superintendent Jonna Turunen said the school had been informed of the plans, but would not give further details pending the investigation.

A Helsinki court remanded the 21-year-old suspect in custody until Nov. 1 on suspicion of planning a "life-threatening crime" over 18 months, but gave no other details.

Finland saw its first school massacre in 2007 when a gunman killed eight people at his high school. A year later another gunman opened fire at a vocational college, killing 10.