Investigators probing the fatal shooting of a police sergeant in a custody suite are working with a “determination to find justice” for their fallen colleague, a top officer has said.

The suspect in the killing of Sergeant Matiu Ratana at Croydon Custody Centre in south London in the early hours of Friday 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.

Sgt Ratana, 54, was originally from New Zealand and joined the force in 1991.

The officer, known as Matt to friends and colleagues, 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.

A keen rugby player, he was head coach of East Grinstead Rugby Club, where he was viewed as an “inspiring and much-loved figure”.

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 are “painstakingly” searching four crime scenes in connection with the killing, including the custody suite where the incident unfolded at about 2.15am on Friday.

Forensic searches are also being carried out in an area of London Road, Pollards Hill, Norbury, where the suspect was initially arrested by officers for possession of ammunition and possession of class B drugs.

Other searchers are taking place at an address on Southbrook Road, also in Norbury, and at a second address on Park Road, Banstead in Surrey, where local police officers from Surrey Police are helping.

The Independent Office for Police Conduct (IOPC), which attended the scene after the shooting, said the 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 by regular officers 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 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.

The Met sergeant is the 17th from the force to be killed by a firearm since the end of the Second World War, according to the National Police Memorial roll of honour.

Ex-policing minister Mike Penning wrote in The Sun that poor attitudes towards officers started with politicians, who he accused of “hampering and undermining normal police officers”.

On Sunday, the Prince of Wales will lead tributes to fallen police officers for National Police Memorial Day (NPMD), honouring those who have lost their lives on duty.