The WA Police Union is demanding answers to why four of its members were arrested and detained for up to 13 hours like “criminals” after last Saturday’s fatal shooting in Carlisle.

Police Commissioner Karl O’Callaghan is understood to have met with the officers today and personally apologised to them for the way they were treated, which the union’s president George Tilbury described as “disgusting”.

According to witnesses, the officers saved the life of lunch bar worker Sheila Tran who was being stabbed in the back by 38 year old Brendan John Lindsay when they opened fire on him.

Mr Tilbury said the decision to arrest the officers had compounded the trauma they were already experiencing and made no sense as all had agreed to cooperate fully with internal investigators.

“The way our members were treated was appalling,” he said.

“WA Police have a lot to answer for because to my knowledge this has never happened before anywhere in Australia and I do not want it to happen again.”

The officers were all released without charge.

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One officer who was covered in Mr Lindsay’s blood as a result of trying to resuscitate him was prevented from cleaning his hands for several hours because of a delay in forensic testing.

Mr Tilbury said the officers — all believed to be in their 20s — were kept isolated in separate rooms for hours at a time and were not told what charge they were being questioned over until the union’s lawyers arrived.

A spokeswoman for the commissioner said police had the power to arrest officers if necessary, but no explanation was offered for the decision.

“The option to use these provisions is a general power which is available to all investigating officers,” she said.

“The incident remains under police inquiry and will also be the subject of a Coronial Inquest.

“As such, WA Police will not discuss details of matters under investigation nor make further public comment.”

But Mr O'Callaghan told 6PR this morning that none of the officers had complained about their treatment.

"They have not expressed concern to me or to their counsellors," he said.

Mr O'Callaghan said that if it was correct one of the officers was crying on the side of the road, they should have been moved to a more secluded area.

"They should be moved. That's not good if they are sitting them and they are seen by the public," Mr O'Callaghan said.

"It has been some years now since we shot and killed someone as part of our duty.

"There's nothing to say the officers haven't acted responsibly and done their jobs so I have been supporting the officers all the way through this.

"But we do have to do an investigation which will be pulled apart shot by shot in the coroner's court."

Shadow police minister Michelle Roberts questioned why it had taken five days for the meetings to take place.

“An innocent party could have been killed if it wasn’t for the actions of these officers, so to treat them like common criminals ... it just beggars belief,” she said.

An officer who had previously worked in the internal affairs unit told The West Australian that he could see no logical reason behind the decision to arrest the officers.

Video of the incident shows a member of the K-9 unit pleading with Mr Lindsay to put the knife down, warning him repeatedly that police were prepared to shoot if he harmed Ms Tran.

Mr Lindsay’s father John saw the shooting and has publicly stated that he bears no ill-feeling towards the officers, saying they were “only doing their job”.