The man who deliberately drove a four-wheel drive into a Melbourne crowd injuring 19 people is a 32-year-old with a history of drug use and mental illness, police say.
The Australian citizen of Afghan descent was driving alone when he crashed a white Suzuki SUV into pedestrians at high speed outside Flinders St Station on Thursday about 4.45pm.
He is in custody in hospital as police continue their "fluid investigation" into what they allege was a "deliberate act".
The man has been named as Saeed Noori by the Herald Sun, citing police sources.
Victoria Police acting chief commissioner Shane Patton said the driver was known to police for "historical assault matters" but was not on bail.
Patton said the man had a "history of drug use as well as mental health issues".
"We understand that he is on a mental health plan and receiving treatment for a mental illness," he said. "We're working to clarify that."
The crash is not being treated as a terror investigation "at this time", despite counter-terrorism teams being involved. Patton said police believed it was "quarantined to a singular incident" rather than part of a larger plot.
"It's horrendous and life-changing for all emergency services who attend," he said. "It's life changing for victims and for those present. Their lives will be changed forever."
Nineteen people were injured in the seconds of chaos, including a 4-year-old boy who is believed to have a head injury. Four are in a critical condition and 15 are reported to be stable. They range from 4 to 83 years old. Some ages are still unknown.
The 32-year-old driver and another 24-year-old man were arrested at the scene. The 24-year-old was seen filming on his phone and carrying a bag of knives. Police later said he was not linked to the attack.
Victorian Premier Daniel Andrews slammed the "act of evil" but praised an off-duty policeman who detained the driver.
"He [the off-duty officer] deserves special praise for the way he instinctively came to the rescue of others, avoiding so much further carnage."
Extensive footage posted online shows a man in a white shirt and blue jeans being dragged from the passenger side of the vehicle while others lie on the ground around it. Smoke pours out of the bonnet of the car.
Footage filmed by one witness showed police kneeling over a man in a white T-shirt and checking his pulse immediately after the car stopped.
One witness told the Herald Sun police were "knocking" the driver and "trying to wake up him up".
"He was pulled out of the car with a gun at his chest. They dragged him out. He looked unconscious," he said.
Prime Minister Malcolm Turnbull sent his "thoughts and prayers" to the victims and the emergency services workers tending to them.
The shocking event came nearly three years after the Sydney siege, which happened on December 15, 2014, and about 11 months after another man drove a Holden Commodore down Bourke St in Melbourne, killing six.
Yesterday's smash coincided with a reported gas bottle explosion at Frankston Bayside shopping centre, which police treated as unrelated.
A 4-year-old boy with head injuries was transported to Royal Children's Hospital in a serious condition. Other victims were taken to the Royal Melbourne and Alfred Hospital, and some were treated in the street.
Witnesses described horrific scenes that included "people flying everywhere" as the carnage unfolded at the corner of Elizabeth and Flinders Sts during rush hour.
The vehicle ran a red light just after 4.30pm, witnesses told local radio station 3AW, before mowing down pedestrians. Bystanders said the vehicle made "no effort to slow down".
Absolute chaos in Melbourne's CBD. Several people run down by a driver on corner of Elizabeth and flinders St @theheraldsun pic.twitter.com/2uBHvEIggX
— Cassie Zervos (@cassiezervos) December 21, 2017
Sue, of Walker's Doughnuts, said: "We could hear this noise, as we looked left, we saw this white car. It just mauled everybody down. People are flying everywhere. We heard thump, thump. People are running everywhere."
Witness Jim said the Suzuki was travelling west along Flinders St at up to 100km/h.
"The intersection was full of pedestrians and he just ploughed through," he told ABC TV. "The only thing that slowed him down was him hitting pedestrians.
"There was no braking, there was no slowing down. Whether it was targeted or whether he had a heart attack, or was drunk, I don't know."
Lauren Lopatko tweeted: "Something just happened outside Elizabeth St tram stop outside Flinders St station. Avoid area if you can. I saw ppl screaming & smoke."
Reddit user TheElderGodsSmile added: "Sister in law said ... he knocked them over like skittles. Bloody awful."
Fright_ posted: "Saw grown men and women crying on Platform 1 of Flinders St station. Overheard phone conversation mentioning car hitting people. Seems like I got there a few mins after it happened. Elizabeth St entrance to Flinders is closed. That big intersection being blocked off by police about 15 mins ago. Heaps of emergency services there now."
Elton Hindoli, who works in the area, said a man who was pulled from the car appeared to be unconscious.
"A girl came into the shop and she was very upset," he said. "It wasn't clear if the driver was hitting them on purpose or not. It sounded like the man who they took out of the car was not awake."
Lachlan Read said the whole thing lasted about 15 seconds.
"He has gone straight through the red light at pace and it was bang, bang, bang. It was just one after the other. The last bang the car stopped," the 20-year-old said.
"There were bodies on the ground and people running up to them — it was mayhem."
Vince was working in a shop at Flinders St Station and rushed to the scene before emergency services arrived.
"There was no screaming, just people lying there," he told AAP, adding that onlookers were quick to lend a hand. "They were just holding them and comforting them."
An hour after the car struck pedestrians, police continued to swarm the area and Flinders St had been cleared and locked down.
Victorian Police said forces and emergency services will stay on scene for the "foreseeable future".
"The crime scene is active and will remain in place for some time. We urge people who can avoid the area, to avoid the area," police said.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (Mon-Fri 1pm to 10pm. Sat-Sun 3pm-10pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.
Winds likely exceeded 289km/h, but damage was limited to trees and power poles.