Watch ITV Meridian's Joe Coshan's report on the trial at Northampton Crown Court
Police officers who stopped and searched a man who then went on to shoot and kill a police sergeant in custody found bullets on him, but no gun.
De Zoysa managed to shoot four bullets, that he made himself, in custody, before being tasered and contained by officers.
The first bullet went through Sgt Ratana's heart, the second passed through his leg, the third hit a wall, and the fourth hit De Zoysa himself in the neck, leaving him brain-damaged.
Northampton Crown Court heard how the weapon, an antique revolver he'd legally bought online, was most likely concealed in a holster, under one of his armpits.
De Zoysa who was 23 at the time of the shooting, was initially stopped while walking home in the middle of the night in South London. Officers explained there had been a lot of burglaries in the area.
At first, De Zoysa appeared shy and nervous before admitting he had non-medical cannabis in his bag.
Reluctant to be searched further, De Zoysa then asked to be taken to the car.
Whilst there, he was told by officers to stop talking and stop reaching into his pocket.
He was searched again and officers found what they believed to be bullets before arresting him.
The moment police find the bullets on De Zoysa
Officers tried to search for the gun but despite administering a check of his person they didn't find it.
The Independent Office for Police Conduct (IOPC) took no further action against the officers.
Former detective Peter Bleksley has experience searching hundreds of people whilst serving in the force for more than 20 years.
He said: "Now I fully appreciate the tragic events that unfolded after this stop and search and I also appreciate that those officers are going to have to carry the burden of what happens around with them quite simply for the rest of their lives.
"However, bullets means gun, which means you're going on the pavement, you're going to be handcuffed behind your back and you are going to be systematically searched. Outer garments are coming off.
"I don't care what restrictions might be placed on stopping and searching people in the street. Bullets equals gun. Find it."
Peter Bleksley speaking about the stop and search
He added: "Let's support those cops who are out there at half past one in the morning.
"Let's empower them to feel like that they're not going to get complained against, they're not going to get disciplined, they're not going to lose their job if they are justified, polite and firm. As firm as the circumstances dictate."
Deputy Assistant Commissioner of Met Police, Stuart Cundy, said: "Conducting a search on the street in the dark in the early hours of the morning will never be 100% perfect but we'll always aspire to be.
"But where there is learning to come from this incident, we've already taken action, we've already rolled out even more hand-held metal detector wands which can assist the searching for where people might have secreted things."
De Zoysa continued to get into the police van and was taken into custody.
CCTV footage shows him fidgeting behind his back whilst in handcuffs to move the revolver into his waistband.
He managed to get through the metal detector at the police station and when asked whether he thought the gun would be found, he said no because the cuffs were metal too.
The events that unfolded next led to the death of Sgt Ratana.
Sgt Matt Ratana was held in the highest regard both in policing and his personal life.
Born in New Zealand, Mr Ratana moved to the UK in 1989, first to Surrey, then to Goring near Worthing.
He was a devoted father, partner and rugby coach before he was killed, whilst doing the job he had loved in the Metropolitan Police for nearly thirty years.
After finishing a training session at East Grinstead RFC, he went straight to a night shift at a custody centre in Croydon on 24 September 2020. He had planned to retire before that Christmas.
At almost 2.10am, speaking to a handcuffed Louis De Zoysa who had just been arrested, when suddenly, out of nowhere, he was shot.
Jay Holt, a friend of Sgt Ratana's, said: "Part of the irony of the way in which he died is that I remember chatting to him when he got that job and he was saying I'm into my last year, it's really safe.
"I'll go into the custody suite and be custody sergeant. And before that, he'd been out in the vans wearing a stab vest, one of these initiatives to battle knife crime."
Sgt Ratana's death sent shockwaves far and wide, his funeral service was streamed around the world.
He is due to be sentenced on 27 July at Northampton Crown Court.
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