A 23-year-old man has been arrested on suspicion of the murder of Sergeant Matt Ratana, the Metropolitan Police says.
Mr Ratana was shot at Croydon Custody Centre in the early hours of Friday 25 September. He was taken to hospital but later died from his wounds.
A 23-year-old man, who had been arrested earlier in the evening for a separate offence and was detained by officers at the centre in Windmill Lane, also suffered a gunshot injury and was taken to hospital.
The Met Police confirmed his condition has since stabilised and is "no longer considered life-threatening".
A statement said: "Therefore, on Friday 13 November, he was considered fit to be arrested on suspicion of murder.
"However, as the man remains in hospital, officers will liaise with the relevant medical and legal teams to make an assessment of when he will be fit for interview.
"Specialist family liaison officers from the Met's Specialist Crime Command are in regular contact with and supporting Matt's partner, his son and wider family. They have been informed of today's arrest."
Speaking after the arrest, Metropolitan Police Commissioner Cressida Dick said: "Matt's loss hit the Met and the wider world everywhere very hard.
"He was a great officer with wonderful qualities and so many people have been impacted by his loss and will miss him a lot.
"You are aware a person was arrested for other offences before that, and after Matt was killed that person went to hospital.
"They have been very unwell but their condition has now stabilised and improved sufficiently for us to be able to arrest him for murder today.
"It's an important milestone and hopefully it brings some tiny comfort to Matt's partner Su, Matt's wider family and everyone who has been affected.
"The investigation is moving on into a dreadful, dreadful homicide."