The suspect who shot a police sergeant dead inside a custody suite had his hands cuffed behind his back when he opened fire, the police watchdog has said.

The long-serving officer, who was said to be close to retirement, was named as Matiu Ratana. The 54-year-old, known as Matt, was killed after the gunman opened fire at Croydon custody centre in south London in the early hours of Friday.

The 23-year-old suspect also shot himself and was critically ill in hospital on Friday evening. Police had been about to run a metal detector over him when he managed, while handcuffed with his hands behind his back, to reach a revolver he had secreted on his body and open fire, shooting the sergeant in the chest and himself in the neck.

The suspect is understood to have been known to the government’s anti-extremism programme.

He had previously been known to Channel, an arm of the government’s Prevent scheme that tries to stop people becoming radicalised, although the incident is not being treated as terrorism-related.

The suspect had been flagged over concerns about both rightwing and Islamist indoctrination, sources said, but nothing of concern was found. Mental health is also a consideration in the inquiry, with the suspect understood to have a learning disability – though whether this was a factor is yet to be determined.

Mystery surrounds how the shooting could have happened inside a supposedly secure area.

The Guardian understands the suspect, who is believed to have Sri Lankan heritage, had been searched by officers away from the police station, and ammunition and cannabis were found

On Friday night, the Independent Office of Police Conduct, which is investigating, said: “What we have established is that the man was arrested for possession of class B drugs with intent to supply and possession of ammunition. The man was handcuffed to the rear before being transported to Croydon custody suite in a police vehicle where he was escorted into the building. He remained handcuffed to the rear and seated in a holding area in the custody suite.

“His handcuffs remained in place while officers prepared to search him using a metal detector. It is at the 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.”

Police officer shot dead in Croydon was ‘much loved’, says Cressida Dick – video

Metropolitan police commander Cressida Dick said Sgt Ratana was known as a “big guy” with a “big heart”.

As well as serving as a police officer, he was the head coach for East Grinstead Rugby FC. In a promotional video on the club’s website, Ratana explains that “having previously competed at a national level, one of our goals is to return to the London leagues”.

Ratana was described as “an inspiration” by a friend who knew him from playing rugby together.

The 27 year-old, who would only give his name as Paul, said: “We all looked to him – on the field or off the rugby field, he always found a way to gee us up. Losing that is big.

Leroy Logan, a former Met superintendent, said there were questions to be answered. “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 said.

“It depends on the calibre of the weapon, because obviously if it’s a small weapon and it can be easily in that person’s clothing, then obviously it brings another question on how thoroughly that person was searched, if at all.

“Those are the things the department for professional standards will look at and the IOPC as well as the investigating officers who will have to look at this thing thoroughly.”

Ratana is thought to be the first officer to be killed in a shooting in the line of duty since PCs Fiona Bone, 32, and Nicola Hughes, 23, in September 2012. They were murdered by Dale Cregan in a gun and grenade attack while responding to a report of a burglary in Greater Manchester.

The prime minister, Boris Johnson, and the home secretary, Priti Patel, had earlier paid their respects to the sergeant. Speaking at the Home Office, Patel said: “I’m deeply shocked and saddened by the tragic killing of the officer in Croydon overnight. All our thoughts are with the officer’s family, friends and colleagues across the Metropolitan police force, but also the policing family across the country.

“This is a sad day for our country as once again we see the tragic killing of a police officer in the line of duty as they’re trying to protect us and keep us safe.”

Lissie Harper, the widow of PC Andrew Harper, who was killed while on duty last year after being dragged behind a car while trying to stop quad bike thieves in Berkshire, said: “This is devastating news. No person should go to work never to return. No human being should be stripped of their life in a barbaric act of crime.

“Another hero has been taken from us in unwarranted violence. They protect us but who protects them? Another life is gone in a disgraceful act that reminds us of the danger our police officers face with every shift they begin.

“My heart is broken for yet another member of our blue line family, and all of his family, friends and colleagues who must now accept a life without him in it.”

At Croydon police station on Friday morning, flowers were laid outside the main entrance. Forensic investigators and police officers could be seen inside.

People living in Croydon were saddened and incredulous at the circumstances of the shooting. One man, who asked not to be named, said he had passed the scene about 2.30am and saw a street filled with police vehicles and blue lights.

Ken Marsh, the chairman of the Metropolitan Police Federation, said: “Officers put themselves in danger every day to protect the public. Sadly, on very rare occasions officers make the ultimate sacrifice whilst fulfilling their role. When that happens we will ensure their bravery and sacrifice is never forgotten.”

The National Police Chiefs’ Council chairman, Martin Hewitt, added: “It is another tragic reminder of the risks police officers take on a daily basis to keep the public safe. Policing is a family and I join my colleagues across the country in mourning the senseless death of one of our own in the line of duty.”

Thanks: theguardian.com/uk