Polish Prime Minister Donald Tusk says a wave of school knife attacks this week was driven by the "glorification of violence" online.

"These are very often spontaneous actions inspired by online activity," Tusk said Sunday at a crisis meeting with security officials. "We are not dealing with any group organized on a big scale."

Three attacks hit schools in the central Mazovian province within four days. On Wednesday, a 19-year-old student hurt nine people at a vocational school. A day later, a 15-year-old killed a 37-year-old female caretaker and hurt two others at a primary school in Leszczydol-Nowiny, north of Warsaw. The BBC reported that a woman was also arrested Saturday after hurting three people while attacking a 17-year-old girl at a school in Warsaw.

Police have since checked about 64,000 schools. By Sunday morning, they had detained 117 people for carrying knives or other weapons, or for threatening attacks, the Associated Press reported.

Tusk said the attacks fit a pattern of "nihilistic violence," a term officials use for attacks driven by hatred of people rather than a political goal. "Some of the people detained are active in the ultraright environment," he said. He said police have seen more online searches for Nazi content and for "mass killers who have become idols online."

The attacks have shaken parents across Poland. Thousands of police officers visited schools on Friday.

The same weekend, police detained a man accused of plotting to kill Tusk and the defense minister. Officials have not linked that case to the school attacks.

What we know: Three school attacks in four days left one person dead and others hurt, and police have detained 117 people in nationwide school checks.
What we don't: What charges the attackers will face, and whether the government will add new rules for schools or online platforms.