Eryk Stasielak, head of the Kraków branch of the National Public Prosecutor's Office, said investigators were examining whether anyone had helped or encouraged the attacker.
"We will investigate every lead, including the Russian one," Stasielak told state news agency PAP.
He said the main focus of the probe was to establish whether anyone had assisted or inspired the attacker, adding that investigators were not ruling out any possibilities.
Justice Minister and Prosecutor-General Waldemar Żurek said on Monday that a possible Russian "hybrid operation" had not been ruled out.
He said investigators had information they needed to check but declined to disclose details, saying it was too early.
Asked whether the attacker could have been inspired by Russian intelligence, Żurek referred to the concept of a so-called "matryoshka system," citing Aquarium, a book by former Soviet military intelligence officer Viktor Suvorov.
The term refers to people who are manipulated without realising they are being controlled, according to Żurek. He cautioned against drawing conclusions, however.
"I don’t want to go that far," he added.
Six people were killed and 16 injured when a 24-year-old man, identified by prosecutors as Michał Oksiński, opened fire on passers-by in Kraków’s historic Main Market Square on Saturday. He drove onto the square in a car and fired an automatic weapon at random.
The attacker, a Polish citizen, died in hospital early on Monday after being shot by police and taken into custody.
A 32-year-old British man and a 51-year-old Austrian woman were among the dead, authorities said.
The four others killed were Polish citizens, three women and one man.
Nationals from Germany, Portugal, Britain, Austria and Russia were among the injured, according to the Polish interior ministry.
Two of the wounded were police officers who confronted the shooter.
Police also found the bodies of Oksiński's parents and 21-year-old sister at the family home in the south-central town of Ostrowiec Świętokrzyski.
Preliminary findings indicate that he may have killed them while they were asleep before driving to Kraków.
The shooting in Kraków's historic main square, a UNESCO World Heritage site and one of Poland’s most popular tourist destinations, has shocked the country.
'Gadsden flag'
While little information has emerged about the gunman and his motives, Interior Minister Marcin Kierwiński said on Saturday that initial findings suggested the attacker might have been inspired by far-right ideology.
The right-wing opposition criticised the statement, with Law and Justice (PiS) party leader Jarosław Kaczyński calling the claims a "blatant lie," the Reuters news agency reported.
National Public Prosecutor's Office spokesman Przemysław Nowak told reporters on Monday that investigators did not yet know what had motivated the killer.
He said police had found a so-called Gadsden flag in Oksiński's room, a historical American yellow banner featuring a rattlesnake and the words "Don’t Tread on Me" in the attacker's room.
The design dates back to the American Revolutionary War in the 18th century.
Nowak said the flag was associated with "various meanings related to freedom or rebellion" but was not widely recognised as a symbol linked to extremism.
Interior ministry spokeswoman Karolina Gałecka said investigators had also found numerous emblems on the gunman's clothing, some of which had already been examined and interpreted.
Żurek said the investigation would continue despite the man's death, with prosecutors seeking to establish whether he had acted alone or been inspired by a group.
Gałecka said Poland’s Central Bureau for Combating Cybercrime (CBZC) had detained five people over online posts praising Oksiński’s actions.
She added that police were on high alert for potential copycats following the attack in Kraków.
Nowak said four firearms and several hundred rounds of ammunition had been found in the attacker's car and among his belongings.
"We secured a very large number of spent casings. In total, at least several dozen shots were fired, possibly even several hundred," he added.
(gs)
Source: IAR/PAP, TVP Info/TVN24, Reuters