A man is being questioned by police investigating the murder of Sergeant Matt Ratana, while the suspect in the shooting remains in a critical condition.
The 54-year-old policeman was shot by a handcuffed suspect in the early hours of Friday morning at a custody suite in Croydon.
The Metropolitan Police officer of almost 30 years has been described as someone who epitomised the role police play in protecting others an - as head coach at East Grinstead Rugby Club - was said to be an “irreplaceable figure”.
ITV News Correspondent Dan Rivers has the latest:
A man arrested at around 2am on Sunday in Norwich, Norfolk, on suspicion of supplying a firearm, remains in custody at a south London police station.
The suspect for the shooting - widely reported to be 23-year-old Louis De Zoysa - has not yet been spoken to by investigators due to his condition.
He remains in hospital.
Police have been searching an address on Southbrook Road in Norbury, south-west London, and another on Park Road, Banstead in Surrey.
A police guard was in place outside Courtlands Farm off Park Road on Sunday, following reports from neighbours of an explosion in the early hours of the previous morning.
Speaking to the PA news agency, one neighbour said: "The blast was at 5.40am and woke up our little boy and we sent him back to bed, but what we’ve heard from other neighbours there was about 20-odd police and they were lined down the driveway."
In the lead-up to Friday’s killing, the shooting suspect had been arrested by patrolling officers for possession of ammunition and possession of class B drugs with intent to supply following a stop and search.
He was then handcuffed behind his back before being taken to the station in a police vehicle.
According to the Independent Office for Police Conduct (IOPC), he was taken into the building and sat in a holding area in the custody suite.
He then opened fire while still in the handcuffs as officers prepared to search him with a metal detector.
No police firearms were discharged in the incident, during which the suspect was also injured, and the case is not being treated as terror-related.
Multiple tributes have been paid to the New Zealand-born sergeant since his death, including from the country’s Prime Minister Jacinda Adern.
Metropolitan Police Commissioner Dame Cressida Dick said he was an “extraordinary person” who was “very good at his job”.
She said his “terrible” death might bring home to people the challenges of police work, helping them to “see us police as who we are – human beings, going to work to help people, to support people and to protect people”.
She added: “Matt was the epitome of that.”
The Prince of Wales, speaking during a virtual service for National Police Memorial Day on Sunday said his death is a reminder of the dangers officers are confronted with.
Charles said: “The dreadful incident in Croydon on Friday is the latest heart-breaking evidence of the risks faced by our officers daily.”
Friends and teammates gathered in silence in East Grinstead to pay their respects as the club flag was flown alongside the New Zealand flag and an All Blacks rugby team flag to honour Sgt Ratana’s love of the sport and his roots.
Police have previously said they are working with a “determination to find justice for our colleague and his family”.