Australian police said there is no evidence of a terrorist link to a car ramming attack on Thursday in central Melbourne that injured 19 people.
Police said the driver was a 32-year-old Australian citizen of Afghan decent who has a history of drug use and mental health issues. The man was known to police for historical minor assault and traffic offences.
"We don't at this time have any evidence or any intelligence to indicate there's a connection with terrorism," said Victoria state police acting commissioner Shane Patton.
The streets outside the city's iconic Flinders Street railway station were crowded with Christmas shoppers late on Thursday afternoon when a white Suzuki SUV ran a red light and sped up to slam into pedestrians crossing the road before crashing into a traffic barrier.
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While police had earlier said that 14 people had been injured, Victorian premier Daniel Andrews confirmed that 19 people had been taken to hospitals. The four listed in critical condition are believed to include a preschool-aged child, Australian media reported.
Mr Andrews described the incident as "an act of evil and an act of cowardice, perpetrated against innocent bystanders".
Witnesses described a horrific scene.
"I was crossing Flinders Street on the way to the train station. I heard an engine rev behind me and heard a thump," one man identified only as David said on ABC TV. He said he turned around and saw "people literally getting thrown into the air as it hit them".
Bystanders rushed to drag the driver out of the vehicle before police arrived to arrest the man.
Early reports said there were two men in the car, but Mr Patton, the police acting commissioner, confirmed the suspect was driving alone.
Confusion had arisen when a second man was seen being apprehended alongside the driver. Mr Patton said this was a 24-year-old man who was seen filming the incident on his mobile phone and found carrying a bag containing three knives.
Mr Patton said that man was taken into custody to determine whether he was connected.
"At this stage we don't believe that to be the case," he said.
The investigation would involve the force's counter-terrorism unit to ensure "that there isn't that connection, and that there is no ongoing threat", he said, but that the attack was being treated as a singular incident. He said police were working with Australian intelligence agency Asio, and with Australian federal police, and that there was no intelligence to suggest the man was a person of interest to those bodies.
"We understand he's on a mental health plan and receiving treatment for a mental illness," Mr Patton said.
He added that while police numbers would be increased around Melbourne in the coming days, there was no increased danger threat surrounding the city over the busy holiday period.
Prime minister Malcolm Turnbull sent a tweet calling the incident "shocking" and said his thoughts and prayers were with the victims and those treating them.
It is the second time this year that Melbourne, regularly ranked one of the world's most liveable cities, has been traumatised by a car attack.
In January, six people were killed and more than 30 were injured when a car was driven up a footpath in Bourke Street, near the scene of Thursday's incident. Police arrested the driver, who was known to police for a history of illicit drug use, family violence and mental health problems. Police said that case was not terrorism-related.