Potential evidence in the investigation into the murder of two Brent sisters was accidentally thrown away.

Police believe Nicole Smallman, 27, and her sister Bibaa Henry, 46, were murdered by someone unknown to them after the sisters were found dead five minutes from the Valley Drive entrance to Fryent Country Park on June 5.

They are believed to have been with a group of 10 celebrating Bibaa’s birthday in the park from around 7pm. People are thought to have left until only Nicole and Bibaa remained at about 12.30am.

They were reported missing late on June 6, and their bodies were found with multiple stab wounds on June 7.

Senior investigating officer Det Ch Insp Simon Harding said a litter-picking volunteer inadvertently threw away their property from Fryent Country Park.

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Officers have spent five days sifting through rubbish at a waste centre to recover items, any of which could contain critical forensic evidence.

Det Ch Insp Harding said: “We had to take a bit of a punt on it, because it could go to any of the rubbish sites and sometimes they are transported between them, so we were lucky that working on where we thought it might go, we picked the right place.”

From blood on the scene, the murderer is believed to have been unknown to the victims and to have incurred a “significant” injury - which may now be bandaged.

Det Ch Insp Harding said there is no clear motive and that police do not yet understand exactly what happened.

“We have eliminated everybody at the party and others from our enquiries and at this stage, I am certain they (the perpetrator) were not known to them (the victims).”

He appealed for anyone with any information to come forward.

He said: “We are still in an important phase of our investigation. It’s fast moving and we are still working on this around the clock but we need the public’s help. People need to sit down and focus on what they remember.”

Police will be conducting extra patrols around the area in light of the killings.

Anyone with information can call 020 8721 4205, via 101 or tweet @MetCC quoting CAD 3160/7Jun.

Crimestoppers take anonymous calls on 0800 555 111.