Four addresses are being searched by detectives investigating the fatal shooting of a police officer in a custody suite in London.

Metropolitan Police sergeant Matiu Ratana, 54, who was known as Matt, was shot in the early hours of Friday when a handcuffed gunman opened fire before turning the weapon on himself.

Deputy Assistant Commissioner Stuart Cundy said a 23-year-old man arrested on suspicion of murder following the incident at Croydon Custody Centre remains in a critical condition in hospital.

"Due to his state, we are not yet able to speak to him," he said during a news briefing at Scotland Yard.

He said the gun used had been recovered from the custody suite.

"We also have the CCTV from that custody suite, which shows the events," he said.

"And we have body-worn video of our police officers who were involved in the circumstances surrounding the arrest of the suspect.