Ex-cop's final eerie act before horror tunnel smash that killed Lily Galbraith while she was on the way to a wedding

A medically retired policeman stopped his black Audi at the entrance of a busy tunnel, revved his engine and accelerated to speeds of 200km/hour just seconds before he ploughed into the back of a car containing two young women.

Ex-cop Bruce Daley, 56, was killed when he smashed into a silver sedan which then crashed into a truck in the Legacy Way tunnel at Mt Coot-tha on Wednesday. 

Young nurse Lily Galbraith, 24, who was a passenger in the silver sedan was killed while the driver Emma McLean, 23, was left in a critical condition. 

Witnesses to the horror crash recall how the Audi stopped at the entrance of the tunnel at about 3.10pm, forcing other drivers to move around it. 

'[The driver] didn't indicate or put hazards on … I indicated around him and kept going … a few people beeped at him,' one witness told The Courier-Mail.

'I went past slowly … he was just sitting there in the car.'

The witness had called his brother on his commute home from work. 

'This car was flying, he went past me and my brother could actually hear it through the hands-free … my car shook,' he said.

'About 80metres in front of me is where the impact happened. I told my brother 'I have to get off the phone' and jumped out.

'All you could see was the smoke and debris everywhere.'

The witness said he and another man checked on Mr Daley, who had 'minimal pulse' and was later declared dead at the scene.

Mr Daley retired from the Queensland Police in 2014.

He had mentioned his mental health struggles in worrying online posts including references to loneliness and suicide. 

Detective Acting Inspector Chris Hansel told a press conference that speed would be a focus in the investigation, which was not a criminal one. 

'This will be a very protracted matter and all elements and angles of this incident will be investigated thoroughly,' he said.

'This includes looking at our fatal five factors, such as any distractions, drink-driving, speeding, fatigue or unrestrained behaviours that were present leading up to or during the incident.'

Inspector Hansel would not comment on whether the ex-cop's mental state would inform the investigation, which is expected to take several months. 

'I'm unable to comment on exact scenarios at this stage. As you would appreciate it is a very complex investigation, a very confronting scene that was experienced by our first responders and members of the public,' he said.

'It would be again irresponsible for me to nominate a specific scenario without the full investigation having been undertaken.'

Ms Galbraith, from Darwin, and Ms McLean, from Adelaide, were in Brisbane for a wedding when their lives were turned upside-down by the crash. 

Ms McLean remains in a critical condition at Royal Brisbane and Women's Hospital  after undergoing surgery on Wednesday evening. 

She was engaged to her boyfriend of five years only a few weeks ago. 

Ms Galbraith, who sadly died as a result of the crash, had worked as a nurse.

The driver of the truck was taken to hospital with non life-threatening injuries.  

The collision shut down the major city tunnel with both inbound and outbound lanes closed for over two hours during peak hour. 

Police want to speak to anyone who saw the black Audi S3 before the crash as well as anyone with information for investigators.