Polish police arrest teens over ‘Bloody Friday’ school attack threats

Polish police have arrested a string of young people, mainly for making threats related to school attacks, they said on Friday, as authorities sought to calm fears spurred by rumours online of a “Bloody ‌Friday” after two stabbing sprees in as many days.

Social media in Poland has been filled with talk of a supposed online challenge that could be behind the attacks, but police on Friday said their investigations had found no evidence of this.

The incidents have prompted a nationwide security...