It is thought he was on their radar in the past, but this has not been confirmed by the Met.
Police watchdog investigators are probing the arrest and how the gun was missed when he was searched.
The suspect was detained by special constables after he was seen 'acting suspiciously' and was taken by van to the custody suite in Windmill Lane just after 2am.
Did you know the officer and want to pay tribute? Get in touch at ellie.cambridge@the-sun.co.uk/tips@the-sun.co.uk or 02077824368
New Covid booking rules are in place and it is understood that the sergeant, thought to have been with the force nearly 30 years and just months from retirement, went to "meet and greet" the suspect.
The shooting is thought to have happened inside the custody suite as the prisoner was about to be assessed and have his temperature and details taken.
The injured officer's colleagues and medics desperately tried to save him, before he was rushed to hospital and tragically died.
Lisa Verrilli, a local cafe worker, said she saw the hero cop several times a week when he would visit for lunch.
The 39-year-old said: “He was only in here two days ago - it’s absolutely heartbreaking. My heart goes out to his family.
“He was a true gent. He was a very proud dad and a devoted family man.
“He’d been in the force for a long time. Despite a demanding job he was always in a good mood. He was happy and cheerful.
“His family meant everything to him. He was always talking to us about them. He was a confident man and I could see him being a brilliant officer.
"He was really keen on rugby. He would always be talking about it. My boys are rugby players so we chatted about that a lot.
“What’s happened is just so awful. The mood here is really sombre. We’re always really moved. “We’ve heard he wasn’t that far away from retiring. It’s just so tragic and awful.”
What we know so far:
- A 'long-serving' custody sergeant was shot and killed this morning
- He was fatally wounded while booking in a suspect to Croydon Custody Centre
- The gunman, 23, turned the weapon on himself and is critically injured in hospital
- He had been arrested on suspicious of possessing ammunition
- The Prime Minister and Home Secretary paid tribute to the killed officer
A player from the rugby club where the tragic officer coached said he had come straight to his final shift from a training session last night.
He said: "He was so vibrant, so bubbly. Larger than life. He was just a great character.
"His wife is lovely. It's a huge loss. If you had a problem you could go and speak to him. He would find a way to help.
"The man was a machine. He was at training last night.
"He used to train with us then he would come to do shift work here in Croydon.
"He would do that week in week out. We start training at 7pm. He would have left at 9, 9.30pm I guess."
The sergeant is the tenth officer to have been killed in the line of duty in the past decade, and the first since PC Andrew Harper was killed by thieves while responding to a burglary in Berkshire in August last year.
PC Harper's widow Lissie Harper wrote Facebook the killing was 'utterly devastating', adding: "What is happening to our world?"
Met officer Stuart James tweeted: "This morning my team and I responded to the worst possible radio transmission from custody, words and scenes I shall never forget.
"The unimaginable happened to our police family. We have lost not only a good skipper but also a real gentleman. One of the best. RIP brother."
And another colleague wrote: "Hard to put into words. All I can say I lost a friend today and know very many more friends did everything they could. Devastated. I'm thinking of everyone affected."
He was a true gent. He was a very proud dad and a devoted family man.
Lisa Verrilli
Tearful colleagues continued to lay flowers throughout the day, with 11 police officers marching towards the custody centre at one point to leave floral tributes with a poem.
Community police officer Jacqueline Kufuor burst into tears after laying flowers outside the centre today in tribute to her colleague.
She said: "You never expect this to happen when you go to work. For him to have been in custody and for this to have happened, it is just so sad.
"He was a very lovely man. He was such a nice man. When he sees you, he would just stand and talk to you. He would ask you about your job and how you're coping and how you are doing out there."
The Independent Office for Police Conduct was notified and will today begin examining the shocking circumstances of the officer's death.
'A VERY LOVELY MAN'
Both former police officers and offenders expressed surprise at the possible lack of checks, with it understood the suspect was being booked in for possession of ammunition.
One man known as MusicBox MB, 33, from Croydon told the Sun Online: “I’ve been in there a few times. Before they take you into the station they search you outside and the van you come in.
“There is no way you can get into that station with a gun, you can't get in there with a tiny bag of weed. 100 per cent they won't have searched him properly, that's why this happened.”
And one local man, 31, who had also been detained in the custody centre, said: "It doesn't make any sense to me. I don't know how this guy has the freedom to start shooting up the place.
"They arrested me at home and still searched me and cuffed me. This guy is walking in there with arms, uncuffed, unsearched."
Asked about how someone could enter the building while armed, former Det Ch Insp Chris Phillips said: "I think police officers are probably less likely to search people now with all the furore that goes on.
"When people get arrested there is a general view that they should be searched before being transported to the police station but that doesn't always happen - and it depends on what the man was arrested for in the first place."
What is the police protocol for searching for suspects?
After a suspect's arrival at the police station and for the duration of their time there, these rules apply to constables and designated detention officers.
Staff are subject to training on searches and refresher courses.
This can include a strip search, although officers are not encouraged to automatically resort to carrying these out unless considered necessary.
Codes of practice require the custody officer to explain the reasons for the search to a detainee and how it will be conducted, as well as ensuring it is done so with "respect and dignity".
Separate powers also allow searches at any other time if a custody officer believes a detainee is in possession of an item which could injury themselves or anyone else, damage property, allow them the tamper with evidence, or escape.
This law also gives officers the power to search a person on arrest and use reasonable force to conduct these.
After a suspect has been searched on arrest "they should not be left unsupervised until they have been presented to the custody officer, who will decide whether or not a further search is necessary".
Meanwhile, under the Police Reform Act 2002, escort officers have the power to search people being taken to or from a police station and seize evidence while in transit from the place of arrest.
Officers must also search vehicles before and after use to make sure items have not been hidden.
Maria Tripi, who lives opposite the police station, said she was woken by the noise of ambulances.
The 66-year-old said: "I was so surprised because when I heard I was worried it could be terrorism or something like that because it's so terrible."
And Labour MP for Croydon North Steve Reed, 46, left a floral tribute at the scene with party chair Mohammed Islam, 46.
Mr Islam was "deeply touched" by the incident as his son Shakz, 23, is waiting for his start date to become a police officer covering Westminster, Fulham and Chelsea.
The officer's colleagues at MPS Croydon lowered their flag to half mast and wrote an emotional tribute, saying: "Today we lost an honourable, brave and dedicated family member."
Other forces around the country also lowered their police flag to half mast as they paid their respects to the sergeant.
'HONOURABLE FAMILY MEMBER'
Forensic officers in white suits were seen emptying their kit out of a car and going into the custody centre.
Uniformed officers stood outside the gates of the building and police tape could just be seen through a window which had the words "police evidence, no mask no entry" on it which was placed across an inside door.
Non-uniformed police officers wept as they places flowers at the scene.
Neil John-Baptiste, 44, a recovery driver, laid flowers at the scene, saying: "It is terrible. I heard the news this morning. I just went and got some flowers. I feel really saddened."
Commissioner Cressida Dick described him as a "long-serving sergeant", and said: “This is a truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances.
"My heart goes out to his family, direct colleagues and friends.
Some of the officers killed in the line of duty since 2000:
Pc Ian Broadhurst, 34, of West Yorkshire Police, was murdered by David Bieber, 38, in Leeds on Boxing Day 2003.
Detective Constable Stephen Oake died during a police raid on a flat in Crumpsall, Manchester, in January 2003.
Detective Constable Michael Swindells, 44, died after he was knifed in the stomach as he and colleagues conducted a search in Birmingham in May 2004.
Pc Sharon Beshenivsky was shot dead when she and a colleague tried to stop armed robbers in Bradford in November 2005.
Pc Ricky Gray was shot in the head by a gunman who then turned the weapon on himself in Shrewsbury in 2007.
Pc Gary Toms, 37, was critically injured confronting suspects in Leyton, east London, on April 11 2009. He died six days later when his life support machine was switched off, 25 years to the day after Pc Yvonne Fletcher was shot dead outside the Libyan embassy in London while controlling a crowd of demonstrators.
Pc Fiona Bone, 32, and Pc Nicola Hughes, 23, were murdered by Dale Cregan in Greater Manchester in September 2012.
Pc Keith Palmer, who was fatally stabbed in March 2017 by Khalid Masood during the Westminster Bridge terror attack.
And Pc Andrew Harper died when he was caught in a tow rope and dragged along country lanes after trying to stop quad bike thieves in Berkshire in August 2019.
“We are currently supporting his family and also have a dedicated team providing support to the officers and those in the custody centre who witnessed the shooting.
"When a colleague dies in the line of duty the shockwaves and sadness reverberates throughout the Met and our communities. Policing is a family, within London and nationally, and we will all deeply mourn our colleague."
A Met statement added the killed officer's family were still being told of the tragedy. No police firearms were discharged, a spokesman said.
It it thought to be the first time an officer has been fatally shot on duty since two cops were killed in 2012 by Dale Cregan.
A source said: "Obviously the police at the station are completely devastated by what has taken place today. A very, very rare thing to have occurred, but unfortunately it has happened."
A woman who lives opposite the custody centre said: "I saw the cars in the middle of the night.
"I was going to the bathroom about 2am. I saw the flashing of the lights."
A shopkeeper told The Sun: “I came and opened up about 7am and there were loads of police cars outside.
“Usually at that time in the morning its really quiet, I didn’t know what happened until I saw it on the news. It's awful.”
Another local added: “It's horrible that it could happen inside the police station as well. That’s Croydon for you.”
The station opened in 2012 with space for over 40 detainees, and was described as a “state of the art” secure facility.
Rev Catherine Tucker, of Holy Saviour Church who's parish covers the custody centre, said: “I am sad for both the death of the police officer but also the perpetrator.
“We are conscious of tensions between young people and police in this area over the past few years and we are actually running a project to try and improve those relationships. Unfortunately I am not really surprised that something like this has happened in Croydon."
This is a truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances.
Cressida Dick
The Prime Minister Boris Johnson tweeted saying: "My deepest condolences go to the family, friends and colleagues of the police officer who was killed in Croydon last night."
And Home Secretary Priti Patel said: “I am deeply shocked and saddened to learn that a Metropolitan Police Officer has been shot and killed in the line of duty.
“My thoughts today are with his family, friends and policing colleagues in London and across the country.
“This morning I spoke to Metropolitan Police Commissioner Cressida Dick to express my condolences and to offer whatever support is needed as this tragic event is investigated.
“This is a sad day for our country and another terrible reminder of how our police officers put themselves in danger each and every day to keep the rest of us safe.”
The Mayor of London Sadiq Khan said he is "devastated" by the news, adding "tragic incidents like this are terrible reminders of the dangers our police officers face every single day".
Ken Marsh, Chairman of the Metropolitan Police Federation:
“The murder of a colleague on duty is utterly devastating news. Officers across London are in shock and sick to their stomachs at the nature of his death.
Ken added: “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.
“All our thoughts - and that of all our members - are with his family, friends and close colleagues at this time. We and all members of the Police Family across the country are all utterly heartbroken at this news.”
“Colleagues involved in the incident will have our full support for as long as is needed.”
Labour leader Sir Keir Starmer tweeted: "Horrific to hear of a police officer being shot and killed in Croydon.
"Our police put themselves in harm's way every day to keep us safe. All my thoughts are with the officer's family, friends and colleagues."
Steve Reed, Labour MP for Croydon North, tweeted: "All of us in Croydon are in shock at this heartbreaking tragic news."
Met Police Chief says the police family are 'mourning a great loss' after custody sergeant shot dead at station in Croydon