Detectives were able to match a cushion found at a refuse tip with a picture taken on the night two sisters were killed, to help identify their killer, a court has heard.
Danyal Hussein, 19, is accused of stabbing Bibaa Henry, 46, and Nicole Smallman, 27, in Fryent Country Park in north London on 6 June last year after making a pact with the devil to kill women in exchange for winning the lottery.
DC David Owen gave details of the search for items cleared up from the park after a post-lockdown birthday party for Ms Henry.
A local resident, Martin Ridgeway, picked up a number of abandoned bags and some rubbish from the area where the picnic had taken place.
He put the discarded rubbish into a black plastic bag and carried it and the other bags to the bin area for collection by the council waste disposal team, sending them a picture on a special app.
Police later searched through the contents of a 40-tonne refuse lorry that had picked up rubbish from bins at the park and taken it to a Veolia refuse centre.
DC Owen said they were lucky that members of the party had taken photos from the evening and "we were able to identify items we were looking for".
One photograph, taken by Ms Henry's friend Nina Esmat, showed the sisters sitting together on a grey blanket with cushions and picnic items around them.
Officers searched through the refuse "item by item" until they found the sisters' belongings.
On 12 June, they found a large blue holdall containing bloodstained grey blankets, an unopened sparkling wine bottle, a selfie stick, and LED colour changing lights.
Bloodstained cushions were found in a separate area, one bearing the words: "Life isn't about waiting for the storm to pass - it's about learning to dance in the rain."
Three cushions, including one with an animal print, were found in a blue rubbish bag with blood on them.
The grey picnic blanket the sisters had sat on was tested and a DNA profile was obtained which matched that of Hussein.
During a search of his home following his arrest on 1 July, detectives found a piece of green card with the words of his agreement to sacrifice women in order to win the lottery, signed in Hussein's blood.
Prints from Hussein's right thumb and right forefinger were on the handwritten document. Nearby were pieces of paper with symbols drawn on them, lottery tickets, and a paper pouch containing more paper with symbols and a pendant on a chain, the witness said.
A blue latex glove and black balaclava were on the floor of a wardrobe, the court heard.
A blue Nike rucksack containing a folding survival shovel was also seized, which matched a rucksack said to have been used by Hussein when he bought a set of knives in Asda before the killings.
Hussein had bought the balaclava and two folding shovels from Amazon for a total of £26.97.
The trial continues.