A man has been arrested in Norwich on suspicion of supplying a firearm, police investigating the fatal shooting of sergeant Matt Ratana have said.
Scotland Yard said the man had been apprehended at around 2am on Sunday and was in custody at a south London police station.
Officers have still not been able to speak to the man suspected of Sgt Ratana's murder because he remains in a critical condition in hospital, a Metropolitan Police spokesperson added.
The 54-year-old was shot dead by a handcuffed suspect in a holding area in a Croydon custody suite, who opened fire as officers prepared to search him with a metal detector.
The suspect, 23, had been arrested during a stop and search for possession of class B drugs with intent to supply and possession of ammunition.
As questions were raised about how the individual was able to bring a gun into the police station, Metropolitan Police deputy assistant commissioner Stuart Cundy said investigators are working with “a determination to find justice”.
Sgt Ratana's death had marked a “dark and sad day for the police family”, he said, adding: “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.”
Police were "painstakingly" searching four crime scenes in connection with the killing on Saturday.
Forensic searches were also carried out in an area of London Road in Pollards Hill where the gunman was initially arrested by officers. These had been concluded by Sunday, the Met said.