A cannabis user who shot dead a custody sergeant while handcuffed in a police cell will be sentenced by a High Court judge today.

Gun fanatic Louis De Zoysa was convicted of murder last month after jurors were shown distressing CCTV footage of the 26-year-old using a legally bought revolver to gun down Sgt Matt Ratana.

Sgt Ratana died in hospital despite the efforts of medical staff after being struck by two bullets in a holding cell in Croydon, south London, in 2020.

The jury which convicted De Zoysa, a former tax office data analyst, was not told a shortened infantry rifle, numerous types of ammunition, a pipe gun, and a dummy launcher were found at his rented property after the killing.

De Zoysa claimed diminished responsibility but was found guilty after a jury decided he pulled the antique weapon’s trigger deliberately to fire homemade bullets, and did not suffer an autistic meltdown.

His three-week trial at Northampton Crown Court was shown slow-motion video of New Zealand-born Sgt Ratana being hit in the chest by the first of three shots discharged by De Zoysa within three seconds.

A second bullet struck the 54-year-old in the thigh before De Zoysa was wrestled to the ground by other officers, as a third round hit the cell wall at Croydon’s Windmill Road custody centre.

De Zoysa, who was living in a flat a farm in Banstead, Surrey, discharged a fourth shot while on the cell floor 16 seconds later, hitting an artery in his own neck and causing brain damage.

The killer, who now uses a wheelchair, has communication difficulties and is being treated at a healthcare unit in Northamptonshire, was arrested in London Road, Norbury, in the early hours of September 25 2020.

A bag containing seven bullets and cannabis were found during a search of De Zoysa’s clothing and body but officers did not discover a holster containing a .41-calibre revolver loaded with six rounds.

During the trial, prosecutors said De Zoysa “retrieved” the weapon from under his left arm, while handcuffed to the rear, as he was being transported to the Windmill Road custody block in a police van.

CCTV evidence suggested he managed to get hold of the gun with his right hand around 16 minutes before the shooting and then took advantage of a vent at the back of his overcoat to hide the weapon.

The court was told that after buying the revolver on the internet three months before the murder, De Zoysa used bullets he had made to test that it worked.

The bullets had been made with parts De Zoysa also bought online, the court heard.

Pre-trial hearings, at which De Zoysa was twice ruled fit to plead despite his communication problems, were told he has no previous convictions.

An examination of his digital devices confirmed his interest in weapons and uncovered material relating to ideologies including right-wing extremism, Islamic extremism and homophobia, all of which he later denied being interested in.

De Zoysa is listed to appear before Mr Justice Johnson for sentence in Northampton at 10am on Thursday.