A blue "noose" was around the neck of schoolgirl Ana Kriegel when her body was found by gardai, a trial has heard.
Three of her fingers were inside the ligature, as if she was "pulling it away", Sergeant Declan Birchall said.
The garda who found Ana's body, Garda Sean White, also gave evidence that he thought he had found "either a mannequin or something terrible".
The jury at the Central Criminal Court was shown a blood-stained concrete block and length of timber, which were recovered from the room where Ana's body was found.
It was also shown a 3D reconstruction of the crime scene.
Two youths, aged 13 at the time, have pleaded not guilty to the murder of Ana (14) at Glenwood House, Laraghcon, Clonee Road, in Lucan, last May 14. Boy A has also denied a charge of aggravated sexual assault.
In his evidence, Sgt Birchall said he was in charge of the divisional search team on May 17, 2018, and had been tasked to search the park for any signs of Ana, who was missing.
He said the team of four was searching Glenwood House and its outbuildings when one of his colleagues said he may have found something in the main house.
Sgt Birchall said he went into Glenwood House through the back door and to a room at the front of the house.
Asked by prosecutor Brendan Grehan SC what he saw, the officer said he saw the "body of a female lying on the floor". She was naked, the court heard.
Sgt Birchall said he couldn't see Ana's face and he believed there was something across it.
When he got closer he realised her hair was covering her face "as if she was thrashing her hair and it covered her face", he said.
Sgt Birchall also noted blood at Ana's nose and her head was tilted back.
There was a ligature or "noose" around her neck and she had three fingers inside it, as if she was "pulling it away".
He checked for signs of life but there were none.
Sgt Birchall said he and his team then left the building to allow gardai to photograph the scene.
In his evidence, Gda White said he was part of the divisional search team which was looking for Ana.
On May 17, Gda White said he entered Glenwood House through the back door. He went through a small porch and into the back kitchen.
He said he then turned left into a "dark room".
Gda White said he "initially thought the body was a mannequin". It was naked and the feet were pointing toward him. He called out but there was no response.
The officer said he took in his surroundings and the smell of dried blood, and realised what he had found.
He called out to Sgt Birchall and the area was later declared a crime scene.
In his evidence, Detective Garda Seamus O'Donnell, from the Garda Technical Bureau, said he attended Ana's post-mortem on the evening of May 17, 2018.
He said Ana was wearing fake nails but only three remained.
Det Gda O'Donnell said a length of Tescon-branded tape was wrapped around her neck.
A long silver necklace with a locket in the shape of a heart was stuck to the tape.
Mr Grehan put it to Det Gda O'Donnell that another witness had described the tape as a "noose".
"Possibly," he said, adding that the tape was wrapped once completely around Ana's neck and was "knotted" or "secured in some fashion".
Det Gda O'Donnell said he returned to Glenwood House on May 18 and a number of items from the scene were labelled and packaged.
A length of timber and half a concrete block were then shown by Det Gda O'Donnell to the court.
Both were bloodstained, the garda said, and a nail was protruding from one end of the timber.
Earlier, Detective Garda Eoin Conway, also from the Garda Technical Bureau, said he photographed Glenwood House between May 17 and May 20.
Going through a booklet of photographs, which was provided to the jury, Det Gda Conway told the court that one was of a piece of a stick or "section of a door frame" and another was of a concrete block.
The block had "blood and hair visible on it", the detective said.
Det Gda Conway also took a number of photographs of exhibits at the photographic section of the Garda Technical Bureau.
He described a picture he took of a black hoodie top with logos on the sleeves and another picture of a strap top which was "torn down the side" and bottom.
Other items of clothing photographed by the detective included a pair of shorts, a pair of "dusty and dirty" leggings which were recovered "beside the body" and a white bra, which was bloodstained on one side.
The trial continues.