Initial indications suggest that the suspect who opened fire in central Krakow, Poland's second-largest city, was influenced by far-right ideologies, Interior Minister Marcin Kierwinski said late Saturday.
Kierwinski told a news conference that police opened fire on the suspect three minutes after receiving the first report of the attack.
He confirmed that six people were killed, including one foreign national, while 16 were injured, including five foreign nationals, two police officers and the suspect.
A British national was among the six killed, according to police chief Marek Boron.
Five of the 16 injured individuals are foreign nationals from Austria, Germany, Portugal, Russia, and the UK.
Boron confirmed that three of the deceased victims were Poles and one was British.
The identities of the remaining two have yet to be established.
Police shot the suspect, who remains in serious condition and is undergoing a second surgery.
Preliminary findings suggest the attacker was influenced by far-right ideologies and held ultranationalist views, although those indications have yet to be verified, according to Kierwinski.
Authorities are also investigating whether the firearms used in the attack came from the suspect's family home.
Prime Minister Donald Tusk said three more bodies, two women and one man, were discovered at the suspect's home, which police continue to search. They appeared to be close family members of the attacker, he added.
"Until now in Poland we haven't had such drastic cases of terrorism ... that's what we have to call this cruel attack on people's lives here in Krakow," said Tusk.
Tusk said investigators were also examining the suspect's vehicle because of fears that it could contain explosives.
He said authorities would swiftly identify and detain any potential accomplices.
The shooting occurred shortly after 3 pm local time (1400GMT), near the Main Market Square, one of Poland's busiest tourist destinations.
RMF FM radio said attackers traveling in a vehicle along Sławkowska Street towards the main square fired at people.
Emergency services responded, with an air ambulance helicopter landing in the square.
Witnesses reported that around 20 shots were fired in the attack.
