Meanwhile, police carried out checks at schools across the country.
On Thursday, a 15-year-old attacked three people at a primary school in Leszczydół-Nowiny, in the east-central Masovia region, including a 37-year-old school employee, who died.
The day before, a 19-year-old stabbed nine people at a vocational school in Ostrołęka, northeastern Poland, including five pupils and two teachers.
He also had a backpack containing a homemade explosive.
Separately, police foiled a probable attack at a school in the north-central town of Sierpc.
A 17-year-old who had threatened to "do something bad" was detained, charged and remanded in custody for two months.
15,000 schools checked by midday
At a joint news conference, Kierwiński said police had checked 15,000 schools by midday, with 24,000 officers involved, and 19 people had been detained.
The operation would continue until all doubts were cleared up, he said.
"There will be no compromise. The safety of the youngest is paramount," he added.
Claims of a supposed "challenge" among pupils emerged after reports that one of the attackers wore a helmet with a camera to broadcast footage.
Kierwiński said analysis of all seized electronic devices had not confirmed such claims.
He added he did not want to give the impression that the threat had passed, but warned against panic and urged restraint and cooperation with law enforcement.
Deputy Interior Minister Czesław Mroczek said police had launched a nationwide operation to strengthen security at schools, and that the ministry was working with the local authorities that run them.
Police were also checking how educational institutions control access by outsiders, he said, and the effort would continue until any link between the incidents was ruled out completely.
The Polish police's Central Cybercrime Bureau (CBZC), he added, would work intensively against attempts to spread disinformation or cause panic.
Education Minister Barbara Nowacka told the conference that 59 schools had been closed nationwide, 45 of them in the Masovia region.
She said the closures were not ordered by the ministry but decided locally, sometimes at the request of school leaders, who were "better placed to judge the mood."
Some institutions offered care and educational activities, Nowacka added.
She also announced more frequent crisis drills, at least once every six months, with training extended beyond fires to cover drone alerts and an attacker entering a school.
(ał/gs)
Source: PAP