A police officer shot dead by a handcuffed suspect at a south London station had moved into custody work because he thought it was safer as he approached retirement, a friend said.

Tributes have poured in for Metropolitan Police Sergeant Matiu Ratana following his death after a 23-year-old gunman opened fire at Croydon Custody Centre in south London in the early hours of Friday.

Investigations are continuing into how the suspect, who had been detained for possession of ammunition and possession of class B drugs, was able to access the weapon.

Sgt Ratana, known as Matt to family and friends, thought working in the custody suite was his “safest option” as he neared the end of his lengthy police career, friend Neil Donohue said.

Mr Donohue told BBC Breakfast: “He thought it was his safest option just to see him through to his retirement and no-one expected this to happen – certainly not within the police cells.”

He described the officer as “the most nicest, generous man you could meet”, and said he was “just a really genuinely nice guy”.

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

Met Commissioner Dame Cressida Dick, who with the Mayor of London, Sadiq Khan, led police officers across the capital in a minute’s silence on Friday, described Sgt Ratana as a “talented police officer”.

He was “big in stature, big in heart, friendly, capable, a lovely man and highly respected by his colleagues”, and leaves behind a partner and adult son, Dame Cressida said.

His partner’s sister told The Sun he was aware of the dangers of being a police officer but saw it as “all part of the job”.

Describing the news of his death as “devastating”, she told the newspaper: “He was dedicated to being a police officer and had almost 30 years of service.

“He knew the dangers of working in London but for him it was all part of the job.”

Sgt Ratana’s cousin Adrian Rurawhe, a politician in New Zealand, said his relative was a “natural-born leader”.

He told Stuff news website: “We’re all devastated that we’ve lost our cousin, but also to hear about the way he has been killed.

“He was awesome, very outgoing and a natural-born leader. Some people have to learn how to be a leader, but it just came naturally to him.”

The suspect also shot himself during the incident at about 2.15am on Friday and is in a critical but stable condition in hospital.

No police firearms were fired and the case is not being treated as terror-related.

He was not regarded a subject of interest by security services, the PA news agency understands, but reports suggest he may have previously been referred to the anti-extremism Prevent programme.

A murder probe has been launched and investigators from the Independent Office for Police Conduct (IOPC) watchdog attended the scene.

The IOPC have obtained CCTV from the custody centre as well as body-worn video footage from the officers present.

These will now be reviewed in the coming days and initial accounts from the officers present will also be taken.

The suspect was arrested by regular officers following a stop and search, then handcuffed behind his back before being taken to the station in a police vehicle.

The IOPC said he was 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.

IOPC regional director Sal Naseem said: “It is at that point that shots were fired resulting in the fatal injuries to the officer and critical injuries to the man.

“A non-police issue firearm, which appears to be a revolver, has been recovered from the scene. Further ballistic work will be required.”

Leroy Logan, a former Met superintendent, said there were questions to be answered around the circumstances which led to the shooting.

“How did that person come to be in the station, whether it’s in the yard or the building itself, and be able to produce a weapon, whether it’s on them at the time?” he told BBC News.

Meanwhile Andy Trotter, former deputy assistant commissioner of the Met and also a former chief constable of British Transport Police, said increasing sentence lengths was not enough to prevent crime against emergency workers.

He told Times Radio: “We are increasingly becoming a very difficult society to police.

“We need to look right across the board on this and just saying we’re going to put the sentences up will not solve that. It’s got to be about prevention and detection, and much more than just that.”

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.