Louis De Zoysa, 25, was still handcuffed when he opened fire at Sergeant Matt Ratana after officers failed to find an antique revolver and holster believed to have been concealed under one of his armpits.

De Zoysa was taken to the custody block in Croydon, south London, after being found with seven rounds of ammunition during a search in nearby Norbury in the early hours of 25 September 2020.

Northampton Crown Court heard how he deliberately fired the weapon without warning into the 54-year-old officer's chest, causing a fatal wound to his heart and left lung.

A second shot hit New Zealand-born Sgt Ratana in the leg, a third struck the cell wall during a struggle with officers, and a fourth hit De Zoysa in the neck, leaving him with brain damage.

De Zoysa denied murder, telling jurors that he did not mean to fire the gun and was having a "panic attack" at the time, while his lawyers argued his responsibility was diminished by an autistic meltdown.

But he was found guilty of murdering Sgt Ratana - a Met officer for almost 30 years and head coach at East Grinstead Rugby Club - on Thursday.

De Zoysa, who had worked for HMRC in Croydon for around 16 months as a data analyst, was living in a flat at a farm in Banstead, Surrey, at the time of the shooting.

He bought the gun, which was legal to own due to its antique status, on the internet in June 2020 and made the ammunition on the farm, the court heard.

Body-worn footage played in court showed De Zoysa being stopped and searched in London Road, Norbury, south London, by officers, who removed items from his holdall and patted him down.

He was then put into the back of a police van and taken to the custody block in Windmill Road but officers failed to find a gun and holster which "were probably concealed under one of his armpits", prosecutor Duncan Penny KC told the jury.

De Zoysa "retrieved" the weapon from a holster under his left arm, while handcuffed to the rear, as he was being transported to Windmill Road in a police van, prosecutors said.

CCTV footage suggests 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 until the attack.

Footage also showed De Zoysa in the holding cell with Sgt Ratana and two other officers involved in his arrest and detention.

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The first three shots were fired within three seconds at 2.09am, while the fourth came around 16 seconds later.

The footage showed officers pulling Sgt Ratana from the cell after the shooting.

Another officer pointed a Taser towards De Zoysa, who was taken to the ground moments after the gun was initially fired.

Sgt Ratana died of his injuries in hospital despite the efforts of medical staff.

His partner Su Bushby said outside court after the verdict: "Today is about justice for Matt.

"His life was taken too soon in the line of duty, doing a job that he loved - a cruel end to a lifetime of service and dedication protecting others.

"Whilst the court case has concluded, the constant feeling of grief and loss continues.

"My love for Matt, my gentle giant will never end. He will never be forgotten."

Metropolitan Police Commissioner Sir Mark Rowley said: "I cannot begin to imagine how difficult this has been for (Ms Bushby) and all of Matt's family and friends. Matt dedicated 30 years of his life to policing.

"He was nearing retirement when he was tragically murdered. He was an outstanding officer who brought joy to his work.

"He treated everyone with respect, with compassion and with good humour. We will ensure he is never forgotten."

De Zoysa, who appeared in court during his trial in a wheelchair, gave evidence in a modified way because of communication difficulties caused by his brain damage.

He nodded when asked if he knew that Sgt Ratana had died and said he felt "sad" about it.

Jurors were told they had to consider whether De Zoysa deliberately fired the revolver and the question of diminished responsibility.