Żurek, who is also prosecutor general, told Radio Kraków on Tuesday that officials now have access to the attacker’s computer, which showed visits to websites about Anders Breivik, the far-right extremist who carried out the massacre on Utøya island in Norway in 2011.
A silencer was also found in the attacker’s car, he said.
On Saturday, a 24-year-old man drove a car onto Kraków’s Main Square and opened fire on random people.
Six people were killed and 16 injured, including two police officers and the attacker, who died in hospital early on Monday.
Police also found the bodies of his parents and sister in the family's flat in Ostrowiec Świętokrzyski.
Żurek’s comments came a day after crowds, including the president and the prime minister, took part in a silent march in Kraków.
Investigators are also examining the symbols the attacker surrounded himself with, including a Gadsden flag, a yellow banner with a rattlesnake.
Żurek said such flags were widely seen among far-right groups during the attack on the US Capitol, but stressed that owning one does not in itself prove someone's views.
Symbols that "may point" to right-wing extremism were also found at the attacker’s workplace.
The assailant left no letter or message, which, according to Żurek, would make the investigation harder.
He added that mental health problems were also an important factor, and that a combination of influences could lead to such an attack.
The evidence also pointed to careful preparation.
The attacker had almost 500 rounds of ammunition and more than a dozen magazines for a semi-automatic weapon, whereas someone learning to shoot would need only two or three.
The Polish justice minister said the silencer was further evidence of planning, noting that neighbours heard nothing when the family was killed in their flat.
Although proceedings against a dead suspect would normally be dropped, prosecutors are continuing the investigation to establish his contacts, whether he acted alone and whether others inspired him.
Żurek said the evidence so far supported claims made in the first hours after the attack by another minister that far-right ideology was a motive.
Those comments were criticised at the time as premature.
"In cases like this, investigators have to check every theory," he stressed.
The minister declined to discuss possible links to political or other groups, saying checks were under way and that he would only disclose evidence he was certain of.
The attack has prompted calls from the defence and interior ministries to review Poland's gun laws.
Żurek, who said he owns firearms himself, backed a debate on tighter oversight.
He argued that all gun owners should undergo regular medical and psychological checks, not only when they first obtain a licence.
He did not favour restricting gun ownership outright, but warned against US-style access to weapons, which he said would lead to mass shootings.
He noted that more than a million firearms are in circulation in Poland.
"I would like what happened in Kraków to be an absolute one-off, from which we draw some lessons," Żurek said.
(ał)
Source: radiokrakow.pl, tvn24.pl