A boy told Garda interviewers that his co-accused told him he was planning to kill schoolgirl Ana Kriegel but he didn't think he was being serious, a trial has heard.
Boy B said Boy A had approached him the month previously and said to him "hey, wanna kill somebody?".
Boy B told gardai he said "no", and told Boy A it was "retarded".
Boy B also said his friend was wearing a "zombie mask" when he assaulted Ana.
Asked why he didn't say anything to gardai when they called to his house investigating Ana's disappearance, Boy B said he had been "scared".
He also said he was afraid of "being framed by [Boy A]" and he was "ashamed" as he had run away and not helped Ana.
The accused, who were 13 years old at the time, have pleaded not guilty before the Central Criminal Court to murdering Ana Kriegel at Glenwood House, Laraghcon, Clonee Road in Lucan on May 14 last year.
Boy A has also denied a charge of aggravated sexual assault.
Yesterday, the jury finished watching video footage of interviews which gardai conducted with Boy B after he was arrested for a second time on suspicion of the murder of Ana.
The interviews took place at Lucan Garda Station on July 7, 2018.
Present during interviews seven and eight were Boy B, his mother, his solicitor Barry Powderly and Detective Garda Donal Daly and Detective Sergeant (now Inspector) Damien Gannon.
Boy B told gardai other friends told him that Boy A had said he was going to kill someone, but he didn't think he was serious.
Boy B said Boy A had also approached him the previous month and said to him "hey, wanna kill somebody?".
Boy B said he told him "no", and that it was "retarded".
Boy B said he had asked Boy A who he was planning to kill and he [Boy A] told him Ana Kriegel. Boy B said he replied: "In your dreams."
Boy B told gardai he thought Boy A was messing, and he knew he wasn't being serious.
This was because he and his friends "joke around all the time" and never mean stuff, he said.
Boy B said that when Boy A asked him to call for Ana on May 14 he [Boy A] told him it was relationship issues.
Gda Daly put it to Boy B that he had to tell the truth.
"I didn't know what Boy A was planning", Boy B said.
He said he just thought it was a friendly request to bring Ana to the park.
Asked why he did not go for help when he realised what was happening, Boy B said he was "scared" and "shocked".
"My brain was frozen, frozen in place. I didn't know what to do," he said.
Gda Daly put it to Boy B that he could have saved Ana.
"I know," he said.
Asked why he did not try to save her, Boy B responded: "I don't know."
Boy B also told Gda Daly he was ashamed for running away and not helping Ana.
Asked why he didn't say what had happened when gardai called to his home investigating Ana's disappearance, Boy B said he was "scared to tell anyone".
He said he "just tried to forget about it" and tried to pretend that nothing had happened.
"You made up stories," Gda Daly said.
"I was scared," Boy B said.
"Scared of what?" Gda Daly asked.
"I was scared of being framed by [Boy A]," he said.
In the final interview with gardai, Gda Daly asked Boy B to "tell me the truth".
Boy B said Boy A asked him to call for Ana, citing "relationship issues".
He initially refused but gave in when Boy A started "whinging".
Boy B said he and Ana walked through the park to the Lucan Road, and towards the house.
He said Ana and Boy A stopped near the house and he walked inside to explore, before he came back out.
Boy B said Boy A and Ana then walked inside the house. He was told to leave, but he didn't want to go, though he didn't tell Boy A that.
He said he then heard "shuffling" and he went and stood at the door of the room Boy A and Ana had gone into.
"I stood still. I couldn't move. I couldn't do anything," Boy B said.
Boy B said Boy A started stripping Ana and once he got to her bra he looked up at Boy B and that was when he ran.
Boy B said that he ran through the field, and as he did so he heard a "faint scream".
Boy B said he didn't know why Boy A had targeted Ana, nor did he know why Boy A had decided on the abandoned house.
Asked why he didn't tell anyone what he'd witnessed, Boy B said that he was "too scared to tell anyone".
Asked why he didn't say anything to gardai, Boy B said he was too scared to admit he didn't help Ana and was too stupid not to realise what was going on.
Boy B also said he didn't recall seeing Ana's blood.
"Ana's injures were not paper cuts," Gda Daly said. "Do you expect us to believe you didn't see any blood?".
Boy B answered: "From my point of view I didn't see any blood."
Boy B said he did not see Boy A attack Ana. "I only saw him flip her and choke her and take off her clothes," he said.
He also said he did not know how Ana's blood was found on a top he'd seen Boy A throw across the room.
"I told you the truth. I told you everything I saw, everything I remember," Boy B said.
Gda Daly put to it Boy B that he brought Ana to the abandoned house.
"You brought the prize. You brought her. You lured her to her death, didn't you?" Gda Daly said.
"No," replied Boy B.
Gda Daly also told Boy B that he had ample opportunity to tell gardai what had happened yet he let "this charade play out in the park and people were out searching when you knew Ana was dead."
"I didn't think [Boy A] would murder her. I didn't want to think that," Boy B said, adding that he kept thinking that it couldn't be real and Boy A wouldn't do that because "it's not like him".
"Can you see why I believe you were involved in the murder of Ana?" Gda Daly asked.
"Yes," said Boy B.
"You're not saying anything to change my mind," Gda Daly added.
"I told you the truth," Boy B said.
"You've told us lie after lie after lie," Gda Daly added.
"As I said before I told you the truth," Boy B repeated.
Earlier, Boy B was questioned about a backpack carried by Boy A. Gardai said they found it in Boy A's home, and it contained gloves, shin pads, knee pads, a snood and a mask.
Gda Daly showed a photograph of a mask to Boy B.
Boy B called it the "zombie mask" because he said that's what Boy A called it.
Boy B said that he had worn it previously and it was "really cool".
He initially said he hadn't seen the mask on May 14 but later asked by gardai if Boy A was wearing the mask, Boy B responded: "Yes, he was wearing the mask."
He added that Boy A was wearing it for the "whole incident".
The trial continues before Mr Justice Paul McDermott and a jury of eight men and four women.