An 18-year-old man has been arrested on suspicion of murdering two sisters in a park in London.
The bodies of Nicole Smallman, 27, and Bibaa Henry, 46, were found at Fryent Gardens in Wembley on 7 June, two days after a birthday party in the park.
Post-mortem tests revealed they died from multiple stab wounds.
Their family has been told in person about the arrest, the Met said. The arrested man from south-London was taken into custody where he remains.
The sisters had met friends in the park at about 19:00 BST on 5 June to celebrate Ms Henry's birthday.
The group had been in an area of Fryent Gardens which was about a five-minute walk from the Valley Drive entrance.
Ms Smallman and Ms Henry are thought to have ended up alone by about 12:30 on 6 June.
Police said they were in "good spirits" and "taking selfie pictures, listening to music and dancing with fairy lights" until at least 01:13.
Calls to them from family at 02:30 went unanswered, police said.
Extensive forensic work has finished at the park but smaller searches in outer areas continue, police said.
More than 1,000 exhibits including property belonging to Ms Smallman and Ms Henry have been recovered and hours of CCTV collected, the Met said.
Det Ch Insp Simon Harding, said: "Given the significance of this development, we visited the family in person today to inform them of the arrest.
"Our thoughts remain with them at this very difficult time."
The arrest comes after two Met Police officers were suspended after "inappropriate photos" were taken of the crime scene.
The unnamed officers were arrested by the Independent Office for Police Conduct (IOPC) on 22 June and have since been bailed.
Talking to the BBC last week, the girls' mother Mina Smallman said the pictures "dehumanised" her children.
The IOPC said the pictures were allegedly "shared with a small number of others", adding the Met was "handling matters involving those members of the public who may have received those images".
The police watchdog is also separately investigating how the Met handled calls from worried family and friends of the sisters after they went missing.