The man who killed six people and wounded 15 others in a suspected far-right terrorist attack in the Polish city of Kraków has died, police said early Monday.

Michał Oksiński, 24, died at 4:30 a.m. local time (10:30 p.m. ET), authorities said. He had been in critical condition since the attack and had undergone several operations.

Police shot Oksiński during Saturday’s midday rampage in Main Market Square, the historic center and major tourist destination in Kraków, Poland’s second-largest city.

Four women and two men were killed. Three of the women and one of the men were Polish, authorities said. A British man and an Austrian woman were among the dead, they said.

Two police officers were in “serious” condition after they were injured along with dozens of others, regional official Krzysztof Jan Klęczar said Sunday. Five foreigners, from the U.K., Portugal, Russia, Germany and Austria, were injured, officials said.

The bodies of Oksiński’s sister, mother and father were found at the family home in Ostrowiec Świętokrzyski, about 90 miles northeast of Kraków, authorities said.

Very little information has come out about Oksiński. Interior Minister Marcin Kierwiński said Saturday that he used an automatic rifle to carry out the attack and that he “was fascinated by far-right ideologies.” The shooter held “very radical” and “ultranationalistic” views, Kierwiński added.

Authorities were still examining the evidence Monday.

The nationalist opposition has questioned the grounds on which the motive was established, with Law & Justice party leader Jarosław Kaczyński saying the claims were a “blatant lie.”

And National Prosecutor’s Office spokesperson Przemyslaw Nowak said at a news conference Monday that investigators did not yet know what motivated the killer.

He said a yellow flag with a snake and the words “Don’t tread on me” on it, known as the Gadsden flag, was found in his room.

First used during the American Revolution in the 18th century, Nowak said, the flag was associated with “various meanings related to freedom or rebellion” but was not widely known as being linked to any form of extremism.

Interior Ministry spokesperson Karolina Galecka said there were many different emblems on the perpetrator’s clothing that had already been partially examined and interpreted.

Separately, government spokesperson Adam Szłapka told Polish broadcaster TVN24 that it was “best to refrain from making comments” about Oksiński’s political views “until the matter is fully clarified and the authorities are in possession of complete information,” even if there were “some initial indications.”

Elsewhere, Justice Minister Waldemar Żurek told Radio Zet that the investigation would continue to establish whether Oksiński acted alone or was inspired by any group.

However, authorities have said they believed the attacker killed his family, possibly while they slept, before he headed to Krakow.

Seven weapons were found in the home, bringing the total recovered to 11, prosecutors said. Four others, including two AR-15 semiautomatic rifles, were found on the attacker or in his car in Kraków. Dozens, if not hundreds, of shots were fired, they said.

However, it has emerged that Oksiński and his father held gun permits and that he had trained at a local shooting club, which said in a statement that he “was never trained in our club in the areas of dynamic, tactical or combat shooting.”

Poland had never experienced such a large mass shooting, and the scenes of bodies and blood strewn across the heart of one of its most historic cities have shocked people in a country that has long viewed itself as safer than its Western European neighbors.