No one has been arrested following the death of 27-year-old Nicole Smallman and 46-year-old Bibaa Henry on Wednesday, police said.
The sisters were found dead at Wembley, in north-west London, Sunday after being missing the day before.
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Mrs. Smallman, of Harrow and Mrs. Henry, of Brent, spent Friday evening at Fryent Country Park with a group of people to celebrate the birthday of the older sister, said the metropolitan police.
People are believed to have left all evening until only Mrs. Smallman and Mrs. Henry stayed at midnight.
Police were called to Slough Lane Park on Sunday afternoon after reports of two women who did not respond.
The sisters were declared dead at the scene.
The cause of death was stabbing, according to a post-mortem examination on Tuesday.
Detectives are following multiple lines of investigation, police said.
“These two women have been stabbed several times and we are working tirelessly to find out who is responsible,” said Chief Detective Inspector Simon Harding, senior investigator.
“Their families have been devastated by their loss and they need and deserve answers.”
He asked anyone in Fryen Country Park on the evening of June 5 to early June 6 to contact the police.
“The area in which the group was located is approximately a five-minute walk from the entrance to Valley Drive Park, leading to a hill area,” adding that it would be “a well-known place to sit and watch London.” ”
“If you were in this area of the evening park from June 5 to Sunday noon, noticed the group or saw something or someone suspicious, please contact us immediately,” he said.
He also asked people who regularly go to the park to come forward if they have seen someone acting suspiciously or have come across things that may not have seemed important at the time, but which could be important to the murder investigation.
Anyone with information is asked to call the police at 101 or tweet @MetCC quoting CAD 3160 / June 7.
Alternatively, people can provide information anonymously to Crimestoppers on 0800 555 111.