Poland has seen a third attack at a school in less than a week.

A 21-year-old woman injured three people at an educational institution in Warsaw on Saturday, the BBC reported. Police said she used a "dangerous object" to attack a 17-year-old girl. Two other people, aged 17 and 24, were also hurt, a Warsaw police spokesman told the Associated Press. None of the injuries was life-threatening.

The woman was arrested. Police did not name the school but said it was open on Saturday.

A Warsaw police spokesperson said they could not confirm whether the attack was linked to the two earlier ones.

Earlier in the week, two stabbing attacks shook Polish schools. One of them killed a female caretaker, the BBC reported.

In response, police visited schools across the country. They checked about 50,000 schools and detained dozens of people, the AP reported via ABC News.

The attacks have alarmed parents and teachers. School violence with weapons is rare in Poland compared with some other countries. Officials have been under pressure to explain how to keep students safe.

Copycat attacks are a known risk after violent events get wide attention, experts often warn. Police have not said whether that played a role here.

The girl hurt on Saturday was taken to a hospital, local media reported.

Police have not released a motive for any of the three attacks. Poland's government is expected to face questions this week about school security, including whether more police or guards are needed at schools.

What we know: a 21-year-old woman injured three people at a Warsaw school.
What we don't: her motive or whether the attack is linked to the two before it.