CHICAGO (CBS) — An SUV slammed into a crowd of pedestrians Thursday in Melbourne, Australia, injuring at least 19 people.

Police arrested the driver and a second man, who was quickly released. Investigators said they believed it was a deliberate act, but there was no evidence to suggest it was an act of terrorism.

The 32-year-old driver is an Australian citizen of Afghan descent with a history of drug use and mental health problems.

“We do not know the motivation and it is still early stages of the investigation,” Victorian State Police Cmdr. Russell Barrett said.

Police said a second man who was arrested was not believed to be connected to the incident, and was assisting police with the investigation. The 24-year-old man filmed the incident, and was carrying a bag of knives, but police said he was not connected to the driver.

The driver plowed a white SUV into a crowd of pedestrians at a cable car stop in Melbourne’s central business district just after 4:30 p.m. Witnesses said the driver ran a red light and showed no signs of slowing down.

Nineteen people were injured, at least four of them critically.

Many of the victims were Christmas shopping at the time.

Police urged people to avoid the scene as the investigation continues to unfold. Authorities said security will be stepped up in the area over the next few days as a precaution.