Flowers were left outside Croydon Custody Centre in south London where a police officer was shot by a man who was being detained in the early hours of Friday morning at the centre in Windmill road.
The Met Police said that no police firearms were discharged during the incident, and the officer's family are being supported by specialist officers.
The suspect was taken to the centre, where he was being searched, before producing a weapon and firing it at an officer and himself during the search.
The officer was treated at the scene by paramedics after the incident at 02:15 BST but later died in hospital of gunshot wounds.
A 23-year-old man who was detained by officers at the scene was also taken to hospital with a gunshot wound and remains in a critical condition.
When asked if 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.
"This goes down the line about handcuffing - do you handcuff, when do the handcuffs get taken off for protection etc."
Dick Cressida, Met Commissioner described the loss of her colleague as "truly shocking and said "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."
Prime Minister Boris Johnson wrote on social media: "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 am deeply shocked and saddened to learn that a Metropolitan Police Officer has been shot and killed in the line of duty.
"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."
Ken Marsh, chairman of the Met Police Federation, 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.
"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 incident has been referred to the Independent Office for Police Conduct which will lead an independent investigation, while The Met continues to investigate the officer's murder.