The suspect was detained with the help of the FBI as part of international cooperation, Poland’s National Police Headquarters said in a statement on Friday.
Authorities said they were alerted to the planned attack on a Canadian school—which was to involve a firearm and a knife—through information shared by US investigators.
Polish officers from the Central Cybercrime Bureau traced online communications in which the teenager allegedly offered to help by sending instructions on constructing explosive devices to a person suspected of planning the attack.
According to police, the girl used an online alias referencing a perpetrator of the 1999 school shooting in Colorado.
Investigators said she also maintained private contacts with users in Turkey and Canada, discussing mass killings.
Police said at least three individuals involved in the correspondence may be linked to an online subculture that glorifies perpetrators of mass attacks, including school shootings.
Messages reviewed by investigators indicate the teenager shared instructions for building an explosive device using a PVC pipe, police said.
During a search of her home, officers seized computers, mobile phones and other digital storage devices.
Police also searched the home of another 17-year-old girl, confirming she had received some of the messages. Authorities said that contact appeared to have been incidental.
The detained teenager faces up to 10 years in prison if convicted.
Police said the case is not isolated and added that officers recently identified a 12-year-old boy who had allegedly been in contact with a person suspected of planning an attack in another European country. His case will be handled by a family court.
Authorities urged parents and guardians to closely monitor children's online activity.
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Source: IAR, PAP, policja.pl