The suspect, a 31-year-old Ukrainian man identified as Ihor M., faces charges of murder, attempted murder and causing grievous bodily harm, prosecutors said at a press conference on Friday, a day after the attack in the town of Jarosław.
The offences carry a maximum sentence of life imprisonment.
The four surviving victims are in stable condition and their lives are not in danger, a spokeswoman for the regional prosecutor's office told reporters.
It remains unclear whether the charges can be formally announced to the suspect on Friday, she said.
He is being treated in hospital for injuries of his own, and experts have been appointed to assess his mental state before he can take part in legal proceedings.
Prosecutors said that if his condition does not allow this within 48 hours of his detention, the formal announcement of charges and any questioning may be postponed.
A request has already been submitted to the court for his pre-trial custody.
Blood tests to check for psychoactive substances have not yet come back, and a translator has been assigned to assist with proceedings.
According to prosecutors, Ihor M. entered Poland from Germany via the Świecko border crossing on the night of 23 September, intending to cross into Ukraine at Korczowa.
He was unable to do so because his vehicle lacked mandatory insurance, and was then driven by other people to Jarosław.
Investigators are still establishing exactly where he got out and how he reached the abbey.
The attack took place in the church and an adjoining Benedictine café, with the knife taken from the kitchen.
Police arrested the suspect outside the church building; he did not resist and complied with officers' instructions.
Investigators have not yet determined why he went to the abbey.
Nearly 20 people who were on the monastery grounds, along with Border Guard officers who had processed Ihor M., have been questioned, with interviews ongoing.
The attack prompted strong reactions from Polish and Ukrainian leaders.
Ukraine's President Volodymyr Zelensky said the perpetrator must be held fully accountable, while Polish President Karol Nawrocki and Prime Minister Donald Tusk called for a swift investigation and firm enforcement of the law regardless of the suspect's nationality.
Tusk also urged the public not to let the attack fuel nationalism or hatred between Poles and Ukrainians.
A local mourning period was declared in Jarosław through the end of the week.
(ał)
Source: PAP