New recruits Constable Josh Prestney and Constable Glen Humphris were killed alongside senior colleagues Leading Senior Constable Lynette Taylor and Senior Constable Kevin King.
Richard Pusey, 41, the man pulled over for allegedly speeding in a black Porsche with drugs in his system, has been charged with failing to render assistance among other offences.
But Deputy Police Commissioner Shane Patton said the focus of the investigation was on the truck driver, who was not known to police.
Mr Bajwa remains in hospital under police guard and is not expected to be medically fit for a police interview for several days. Police have not described the nature of his medical condition, but said it was not a result of the accident.
The crash is being jointly investigated by the homicide squad and the major crash investigation unit.
“We do have a fully open mind on it, that’s why it’s pretty much a dual investigation,” Mr Patton told radio station 3AW on Thursday.
“We haven’t ruled anything out at this stage, pivotal to that is obviously speaking to the truck driver.”
Mr Patton said the driver remained under police guard because he was suspected of serious offences, including whether he may be charged with culpable driving resulting in the deaths of four people.
“For us, the focus is very much, though, on the driver of this truck.”
Police Commissioner Graham Ashton and Deputy Commissioner Patton have both rubbished reports of the truck driver being pulled over earlier in the day as rumour.
Mr Bajwa’s Cranbourne home in Melbourne’s outer south-eastern suburbs was raided on Wednesday night.
“What was found there is still the subject of ongoing investigation,” Commissioner Ashton said on Thursday.
A blood sample for drugs and alcohol was taken on Wednesday night.
The truck involved in Wednesday’s crash bore the logo Connect Logistics, a Sydney-based trucking company whose website has shut down in the aftermath of the crash.
Connect Logistics delivers dairy and other refrigerated items, according to its now-closed website. Despite being based in Sydney, old job listings show a truck driver was hired in Melbourne in 2017.
The company and its managing director have not returned phone calls from The Age.
Mr Bajwa’s family has not returned phone calls.
Rachel is a breaking news reporter for The Age.