BELGRADE and LONDON -- Prosecutors in Poland formally opened an investigation on Sunday and intended to treat the Krakow attack that killed at least six people as a terrorist act, Prosecutor Tomasz Waszczuk told ABC News.

Asked about the motive, he said: "The findings so far point to exactly these motives behind the attack." At least six people were killed after shots were fired in a market square in Krakow, Poland, on Saturday, according to regional police. The suspected shooter was shot and wounded by police, authorities said.

Three other people were found dead inside the home of the suspected shooter, Prime Minister Donald Tusk said on Saturday.

Polish police confirmed to ABC News that the three people found dead in the alleged attacker’s apartment in Ostrowiec Swietokrzyski were his 53-year-old father, 52-year-old mother and 21-year-old sister. Counterterrorism officers spent the night searching the apartment because they were worried he might have left explosives behind. The operation finished this morning.

Waszczuk said prosecutors are preparing six murder charges in connection with the people killed in Krakow. That number could rise to nine if investigators establish that the attacker was also responsible for the deaths of his parents and sister.

The alleged 24-year-old attacker survived the night but remains in critical condition in hospital, officials said. Police spokesperson Katarzyna Cislo told ABC News that he has undergone several operations and that despite everything doctors have done to save his life, his condition remains critical.

He was shot during the police chase and is under constant police guard, but has not yet been formally detained because of his medical condition, law enforcement said. Prosecutors are waiting for doctors to say when he is well enough to be questioned.

Asked whether anyone else was involved, Waszczuk said: "For now, the evidence collected does not indicate that other people were involved. Of course, we will continue to check this." Investigators are also looking into the attacker’s background and examining evidence from several locations.

The police spokeswoman also told ABC News that both injured police officers survived the night. One was brought out of a medically induced coma this morning, while the other remains in a medically induced coma in a relatively stable condition. She could not confirm which officer had been woken up.

Police also confirmed that the injured included 11 people from Poland and five foreign nationals, including one each from Austria, Portugal, Germany, Britain and Russia. Six hospitals in Krakow confirmed those nationalities to ABC News. Two of the injured Polish nationals have now been discharged.

Police confirmed that a 32-year-old British man was among those killed. One of the dead women has still not been identified, so police cannot yet say whether any other foreign nationals were killed.

Police have also appealed for AB Rh+ blood donations for Senior Constable Jakub Kulik, who was seriously wounded while confronting the attacker and is fighting for his life in hospital.

Preliminary findings suggest the gunman was allegedly "influenced by far-right ideology," Polish Interior Minister Marcin Kierwinski said at a press briefing on Saturday.

Krakow’s main square has been closed again this morning as police continue their investigation at the scene. Cislo said she could not say how long it would remain closed.

Poland declared that it would enter a two-day period of mourning beginning on Sunday. Flags were to be flown at half-mast.