A Metropolitan Police officer died after he was shot at a police station by a man who then turned the gun on himself.
The victim died in hospital after the gunman, who was being detained, opened fire at Croydon custody centre in south London during the early hours of Friday.
The 23-year-old murder suspect is in a critical condition in hospital with a self-inflicted gunshot wound after he was held at the scene.
Scotland Yard said no police firearms were fired during the incident at around 2.15am.
A murder probe has been launched and investigators from the Independent Office for Police Conduct (IOPC) watchdog are on the scene.
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.
“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.”
The officer is thought to be the first 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 Met Police officer 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.
Unarmed Pc Keith Palmer, who was stabbed in March 2017 by terrorist Khalid Masood during the Westminster Bridge attack, was the last Met officer to be killed in the line of duty.
Met Commissioner Cressida Dick said: “This is a truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances.
“When a colleague dies in the line of duty the shockwaves and sadness reverberates throughout the Met and our communities.”
Ken Marsh, chairman of the Met Police Federation, which represents rank-and-file officers, said: “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.”
The roll of honour includes Pc Andrew Harper, who 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.
The Thames Valley Police officer’s three teenage killers were cleared of murder but convicted of manslaughter after an Old Bailey trial.
His widow Lissie Harper, who is campaigning for a change to the law which would see all those convicted of killing emergency workers receive a life sentence, described the latest death as “utterly devastating”.
“What is happening to our world,” she asked in a Facebook post.
Prime Minister Boris Johnson was among those who reacted to the news on Friday morning, tweeting: “My deepest condolences go to the family, friends and colleagues of the police officer who was killed in Croydon last night.
“We owe a huge debt to those who risk their own lives to keep us safe.”
Home Secretary Priti 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 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.
Mayor of London Sadiq Khan added: “I am devastated by the news a Metropolitan Police officer has lost his life.
“I was informed of this tragic incident by the Commissioner this morning, and my heart goes out to the family of this brave officer, who has paid the ultimate price for helping to keep Londoners safe.”
The incident has been referred to IOPC, whose regional director Sal Naseem said the watchdog’s investigators were at the scene of the shooting.
The Met is leading the murder investigation and the force said his family are being told.
Forensic officers in white suits were seen entering the police station.
Local resident Wilhemina Dew, 45, said: “It makes me feel scared.”
She added: “Whatever the reason, whatever the tensions shooting the police is unacceptable because we all need to feel safe.”
Reverend Catherine Tucker, of the Holy Church Croydon parish, said: “The action taken against the police is really unacceptable but I also feel sorry for the perpetrator.
“Sadly, I am not surprised there has been a shooting in Croydon.”
She added: “There are tensions between the police and young people particularly in relation to stop and search and the way the police relate to the community.”