The Helsinki police have detained a 20-year-old girl, who is suspected of planning a mass killing of students in one of the city schools, Lenta reports, citing Yle.fi.

The law enforcement officers carried out an operation aimed at neutralizing the plotter as early as last week, on September 15, but the information about her became known only now. The investigation is said to possess hard evidence to support her guilt.

“The preparation to commit the crime was carried out systematically and for a long time,” the press-release of the Helsinki Police Department says.   

The law enforcement authorities don’t mention which educational institution was to turn into an object of the attack. According to the police representative, the details are not disclosed in the interests of the investigation.