The attack took place at about 11:05 a.m. at Vocational School Complex No. 4, where the suspect, a student at the school, attacked five students, two teachers and two other people, prosecutors said.
The victims were taken to hospitals in Ostrołęka and nearby Łomża. Their lives were not in danger, according to authorities. The suspect also injured himself and was taken into police custody.
National Public Prosecutor Dariusz Korneluk said the explosive device, which was brought into the school in the suspect’s backpack, had almost detonated.
According to information provided to prosecutors in Ostrołęka, smoke began coming from the backpack after it was taken into a computer classroom.
A teacher and students threw the backpack out of a window, which may have prevented the device from detonating, Korneluk said.
Bomb disposal experts secured the device, and prosecutors said the investigation into an attempted murder would be expanded to include the explosive device.
Korneluk said the evidence indicated that the suspect had acted intentionally and was prepared for the attack, but cautioned that it was too early to draw definitive conclusions.
The suspect, identified by police only as Jakub Z., was arrested soon after the attack near a city beach, not far from the school. He remains under police supervision at a hospital.
Police are investigating whether the knife used in the attack was also used by the suspect to injure himself while fleeing. His backpack has been secured and will be examined by bomb disposal experts, reporters were told.
Police said the suspect had previously been recorded as a suspect in a drug possession case, but that case was discontinued.
Regional governor Mariusz Frankowski said the suspect was a Polish citizen, addressing what he called speculation in the media about his identity.
Ostrołęka Deputy Mayor Elżbieta Mierzejewska-Nicewicz said the 19-year-old had been receiving specialist care. She said more than 500 students attend the school and that all those affected by the attack would receive psychological support.
Education Minister Barbara Nowacka said anyone needing psychological assistance, including students and teachers, would receive it.
The school was closed after the attack, while Ostrołęka Mayor Paweł Niewiadomski ordered schools in the city not to allow unauthorised individuals to enter.
Prosecutors and police officers were working at the scene, questioning witnesses and examining the school. Four psychologists were also providing support there.
Students at schools across Ostrołęka were scheduled to speak with specialists on Thursday about the attack.
Regional Education Superintendent Wioletta Krzyżanowska called for calm and support for the entire school community. She said teachers had responded appropriately during the attack and that staff training had helped them deal with the emergency.
"Under new regulations, schools are required to devote one hour a month to safety-related activities with students," Krzyżanowska said.
"We are deeply moved by what happened. There is no justification for violence,” Education Ministry spokeswoman Ewelina Gorczyca told Polish state news agency PAP.
"The most important things now are safety, assistance to those affected and support for the entire school community," she added.
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Source: IAR/PAP, TVP Info, TVN24