Police say a teenager entered the Cornwall Centre on Tuesday evening armed with a knife and began randomly attacking people who were nearby.
Deputy police chief Bob Morin says police don't know what prompted the attack.
"This was a very spontaneous, unpredictable event," Morin said Wednesday at a news conference at police headquarters.
"As we conduct our investigation we are trying to determine what the motivation may be. Of course, we can't talk very much about the investigation while the matter is before the courts, but by and large we have no idea what motivated this event."
Morin added there's "no indication at all of any gang involvement, gang initiation or any gang related activity to this incident."
Two police officers who happened to be at the mall as part of their regular downtown patrol heard a commotion and were quickly on the scene. They told the young man to drop the knife, he did and was arrested.
The teen was briefly in court Wednesday and will be held in custody until his next appearance April 29.
Witnesses say the attack was sudden and that the attacker was wearing a red bandana.
Passersby snapped pictures of the carnage, including one posted to the Internet showing a young man lying in a pool of blood as he was being attended to by emergency workers.
All four victims are male. Police say none of the injuries is life-threatening.
Morin said he can't recall another incident like this in 33 years of policing and tried to reassure the public that Regina is a safe city.
"It's rare and it's uncommon," he said.
"I just urge people not to be concerned about their safety because the public safety system in this city works and I think an indication of last night we had a police response almost immediately."
Mayor Michael Fougere offered his condolences to the victims on Wednesday and commended police for their speedy response.
He also reassured citizens that the city's downtown core is safe.
"We have a safe community. We have a safe downtown," he said. "You never want to see this happen anywhere at any time. It’s a tragedy and our hearts go out to those victims that are now in hospital or in recovery."
Dianna Holigroski, who works in the food court at the mall, said she would never expect such a thing to happen.
"It's scary," she said, adding that violence at the mall seems to be getting worse. "It's, like, that could have been me."
Marianne Phillips said she usually stops at the mall on her way to and from work.
"It was quite unsettling," she said. "It actually looked to me almost like a mass shooting that we see on TV."
-- with files from CJMESuggest a correction