Chief Justice Helen Murrell formally discharged the ACT Supreme Court jury on Tuesday, after jurors could not reach a unanimous decision after deliberating for about 21 hours, across almost three days.

A jury representative told the court on Tuesday that the group had “reached an impasse” on whether Alex Ophel, 19, should be held criminally responsible for the attack – which left two students with broken arms and another two people with fractured skulls – on August 25 last year.

The court heard Mr Ophel would face a retrial, which could not be held until next year.

Mr Ophel pleaded not guilty by reason of mental impairment to five counts of attempted murder.

During the three week trial, the court heard Mr Ophel launched the unprovoked attack at a statistics tutorial at the Australian National University about 9.15am.

The prosecution argued Mr Ophel did not meet the threshold for the defence of mental impairment - which is that he could not reason with a moderate degree of sense and composure about whether the conduct, as perceived by reasonable people, was wrong - as he had shown a high degree of reasoning and control in the way he planned and executed the ambush.

The defence claimed the evidence showed it was abundantly clear Mr Ophel was both mentally ill and mentally impaired on the day of the attack.

Prosecutor Shane Drumgold, in his closing address last week, said psychiatrists at the Canberra Hospital, where Mr Ophel was sent after the attack, could only say the defendant had been "prodromal", or at an early stage of mental illness, at the time.

Mr Drumgold warned the jurors to approach the psychiatric evidence with extreme caution as diagnoses were reliant on the truthfulness of what the person said, and therefore could be fragile.

He argued the diagnoses required looking back and cherry-picking behaviour to explain the attack as well as what the person reported about themselves.

The prosecutor said a person might provide a psychiatrist with certain information in order to produce a desired diagnosis.

Defence barrister John Purnell, SC, said expert evidence revealed Mr Ophel had been mentally impaired at the time of the alleged offence.

Mr Purnell pointed to evidence that hospital mental health notes recorded Mr Ophel had at one stage described his hand turning green and trying to strangle him.

The treating psychiatrist had given evidence that Mr Ophel was delusional at the time of the attack and that it was probable he had schizophrenia in the weeks and months preceding it, Mr Purnell said.

The silk said the arresting officer had described his client as dazed and not fully aware of his surroundings, talking in nonsensical way about a tree, and was licking his lips and poking his tongue out.

Mr Ophel was remanded in custody and Chief Justice Murrell ordered the matter be listed for mention later this month.