(Reuters) - A British police officer was shot dead in the early hours of Friday by a man who was being held at a police station in south London. Police said the incident occurred at about 2:15 a.m. while the man was being detained at the Croydon Custody Centre, where arrested suspects are processed. The officer was treated at the scene for injuries but died in hospital. The 23-year-old suspect is thought to have then turned the gun on himself. He was taken to hospital with a gunshot wound and is in a critical condition. Officers did not open fire. London police chief Cressida Dick said the incident was “truly shocking” but was not being treated as terrorism. “Our investigation is led by homicide investigators from the Specialist Crime Command,” she said in a statement. “We are not treating it as a counter-terrorism incident. We are doing all we can to establish a motive for the murder.” Early indications were that the suspect shot himself, she had said earlier. Police named the victim as 54-year-old Matt Ratana, a custody sergeant who was originally from the Hawke’s Bay area of New Zealand. A former London Irish rugby player, he had been in the London police force since 1991. The incident has been referred to the Independent Office for Police Conduct (IOPC) which said the suspect had been arrested for possession of drugs and 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,” it added in a statement. The handcuffs remained in place while officers prepared to search him using a metal detector, the IOPC said. At that point, shots were fired. Prime Minister Boris Johnson said: “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.” The killing of an officer is a rare event in Britain where the vast majority of police remain routinely unarmed. The BBC said only 16 other London police officers had been killed by a firearm since World War Two.