A man has been arrested in Norwich in connection with the investigation into the murder of a Metropolitan Police sergeant.
Sergeant Matt Ratana was shot by a handcuffed suspect in the early hours of Friday at a custody suite in Croydon.
A man was arrested in Norwich on suspicion of supplying a firearm.
Scotland Yard said the man had been arrested at around 2am on Sunday and was in custody at a South London police station.
The suspect for the shooting remains in a critical condition in hospital.
The 23-year-old, who was handcuffed at the time of the shooting and also shot himself, had still not been spoken to by officers on Saturday evening due to his condition.
Investigators probing the fatal shooting are working with a “determination to find justice” for their fallen colleague, a top officer has said.
Metropolitan Police deputy assistant commissioner Stuart Cundy said Sgt Ratana’s death had marked a “dark and sad day for the police family”.
He added: “Everyone working on this investigation, from the forensic specialists to the local officers holding the cordons, does so with a heavy heart but a determination to find justice for our colleague and his family.”
He said police were “painstakingly” searching four crime scenes in connection with the killing, including the custody suite where the incident unfolded at about 2.15am on Friday.
Searches in Pollards Hill, Norbury, where the suspect was initially arrested by officers for possession of ammunition and possession of class B drugs on Friday, have finished, police said.
Other searches are taking place at an address on Southbrook Road, also in Norbury, and at a second address on Park Road, Banstead in Surrey, as well as forensic examinations on the custody suite where the incident unfolded in Croydon Custody Centre at about 2.15am on Friday.
The Independent Office for Police Conduct (IOPC), which attended the scene after the shooting, said the murder suspect had been taken into the building and sat in a holding area in the custody suite, then opened fire while still in handcuffs as officers prepared to search him with a metal detector.
He had earlier been arrested following a stop and search, then handcuffed behind his back before being taken to the station in a police vehicle.
No police firearms were fired in the incident, and the case is not being treated as terror-related.
Deputy assistant commissioner Cundy, who is leading the investigation, said a gun had been recovered from where the shooting happened, and that CCTV and police body-worn footage is being reviewed and will be considered alongside accounts from officers.
Sgt Ratana, 54, known as Matt to friends and colleagues, was originally from New Zealand and joined the force in 1991. He leaves behind a partner and a grown-up son.
Multiple tributes have been paid including from Met Commissioner Dame Cressida Dick and New Zealand Prime Minister Jacinda Ardern.
He had moved into custody work because he thought it was safer as he approached retirement, a friend, who described him as “a really genuinely nice guy”, said.
He is the eighth police officer in the UK to be shot dead in the last 20 years and the first to be murdered by a firearm in the line of duty since Pcs Fiona Bone, 32, and Nicola Hughes, 23, in September 2012.
Also on Sunday Met Commissioner Dame Cressida Dick paid tribute to him as an “extraordinary person” who was “very good at his job”.
She spoke after a morning wreath-laying at the National Police Memorial in central London, which was also attended by Mayor of London Sadiq Khan and Home Secretary Priti Patel to mark National Police Memorial Day (NPMD).
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