One of the two boys accused of murdering teenager Ana Kriegel "lured" her to a derelict farmhouse then watched "voyeuristically" as the other sexually assaulted and murdered her, the Central Criminal Court has heard.
Investigating gardaí found a zombie mask, shin guards, knee pads and gloves in a backpack worn by the boy who carried out the attack, the prosecution alleged.
Prosecutor Brendan Grehan SC made the comments in his opening speech to the jury.
Mr Grehan also said that Ana was last seen alive in the company of one of the accused.
The youths, who are now 14, have pleaded not guilty to the murder of Anastasia Kriegel at Glenwood House, Laraghcon, Clonee Road, Lucan, Dublin, on May 14 last year. One of the boys, Boy A, has also denied a charge of aggravated sexual assault.
In his opening address to the jury, Mr Grehan said that at 1pm on May 17 gardaí, who had been looking for Ana, searched a derelict building where they found her body.
She had been reported missing three days earlier by her mother and father.
Ana was found naked, except for her socks, and there was a ligature around her neck.
Mr Grehan said Ana had obvious head injuries, the scene was bloody and blood was splattered around the room.
He said gardaí began a murder investigation, and Professor Marie Cassidy conducted a post-mortem, which found Ana suffered severe and extensive injuries to her head and neck, which caused her death.
Mr Grehan said it is alleged that Ana was last seen by her family around 5pm on May 14, when she left her home as a result of a visit by Boy B.
She left in his company and was last seen at a public park with him.
Having failed to contact Ana, her family made various attempts to locate her before going to Lucan garda station.
The gardaí responded immediately and ascertained Boy B's address, which was not known to the Kriegels.
They arrived there and spoke to Boy B and his mother. The boy gave the first of many different accounts, telling them he called to Ana and they went to the park where he last saw her. Significantly, he did not mention his friend, Boy A, Mr Grehan alleged.
Gardaí interviewed Boy B again and at that stage he said he had called to Ana on behalf of his friend Boy A.
Boy B said they both met her at the park and told gardaí he knew Ana was interested in Boy A. He said the suggested purpose of the meeting was so Boy A could tell Ana that he was not interested in her.
There was no suggestion by Boy B that Ana was told this, but that she was told Boy A wanted to meet her.
Boy B claimed that after a short conversation in the park between Ana and Boy A, she went one way and the boys went the other and separated, leaving the park.
When contacted, Boy A had a "somewhat different" account, saying he met them in the park, but he did not say there was an arrangement.
When asked when and where they met Ana, divergences began to emerge in what they said, Mr Grehan said.
Beyond the fact that she was missing and not answering her phone, there was no other suggestion of foul play at that stage. CCTV was sought and obtained and Ana was seen on it with Boy B, heading towards the park.
Through mobile phone providers, gardaí managed to "ping" the last activity on Ana's phone. It pinged in the "general area" at 5.32pm.
A number of people gave accounts of seeing Boy A limping, with blood on his face and clothing, not long after 5.32pm. Boy A later told a friend and his parents he had been attacked in the park by two older men whom he had fought off.
He repeated this account to gardaí and a park ranger, and made a statement including references to having seen Ana in the park and that he was mugged a short time after.
"It is the prosecution's case that (Boy A) came by these injuries in a struggle with Ana in which he sexually assaulted and killed her and this was a made-up story," Mr Grehan said.
There was a "compelling" forensic case based on "subjective scientific evidence" by way of DNA, Mr Grehan said.
It was alleged Boy A was connected to the scene by male DNA found on Ana's neck and on the tape used as a ligature on her neck, as well as semen found on her top.
Various items were found in Boy A's home that were connected to him and also connected to the alleged murder.
This included the contents of a distinctive backpack he was seen carrying in the park containing "gloves, kneepads, shin guards and perhaps most chillingly, a zombie-type mask".
"There is no explanation consistent with innocence to explain these objective scientific facts," Mr Grehan told the jury.
The prosecution's case against Boy B was that he aided and abetted in the murder of Ana by assisting and helping Boy A, knowing what was going to happen.
It would be alleged he did this by luring her from her house to meet Boy A, knowing that she would want to meet him and knowing that meeting him "in some romantic way was not the purpose at all."
It was alleged he knew they were going to meet Boy A in a "dirty, dark, derelict house that he or (Boy A) made sure was empty before entry".
It was alleged he knew preparations had been made and remained, "voyeuristically watching her murder and sexual assault and participating in the cover-up afterwards".
The prosecution would say he lied repeatedly.
The trial continues.