A murder accused asked a garda interviewer "are you joking me?" and "are you actually being serious?" when told that Ana Kriegel's blood had been found on his boots, a trial heard.
The Central Criminal Court also heard that Boy A told gardai that CCTV footage of a male, who they believe is Boy A, might be "one of the lads who attacked me".
When asked by gardai during interviews if he was in the room where Ana's body was found, Boy A said "no".
Two youths, Boy A and Boy B, aged 13 at the time, have pleaded not guilty before the Central Criminal Court to murdering Ana Kriegel (14) 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, Detective Garda Marcus Roantree said he arrested Boy A on suspicion of murder at Clondalkin Garda Station at 8.07am on May 24, 2018, and he was detained for questioning.
Over the next two days, gardai conducted six interviews with Boy A.
Present during the interviews were Boy A, his father, his solicitor Donough Molloy, Det Gda Roantree and Detective Garda Tomas Doyle.
Prosecutor Gerardine Small BL led Det Gda Doyle through the interviews with Boy A.
In interview one, Boy A told Det Gda Doyle he was interested in drawing.
During the second interview, Det Gda Doyle said he showed Boy A some CCTV clips.
Ms Small said the jury had already seen the footage.
When shown a clip of two males, Boy A told Det Gda Doyle "they look like the lads that beat me up".
In the third interview, Boy A was shown footage of a male walking in the park, and wearing gloves and a backpack.
Asked if he had anything to say about that individual, Boy A told Det Gda Doyle: "I think that might be one of the lads who attacked me."
Det Gda Doyle told the accused that gardai believed he was the male in this CCTV footage, and in several other clips. Boy A denied it was him, the court heard.
He said a witness statement he previously gave to Sergeant John O'Keeffe was "the truth".
Det Gda Doyle said he then showed a photograph of Boy A's boots to him. Det Gda Doyle told him they were examined and Ana's blood was found on them.
"Are you joking me?" Boy A asked.
"No," said Det Gda Doyle.
"Are you actually being serious?" Boy A responded.
Det Gda Doyle told him he wouldn't joke about something like that.
Boy A then asked if he could get some air. His solicitor asked Boy A if he was feeling sick, and he was handed a glass of water.
Det Gda Doyle then told Boy A he wanted to be clear this was "serious and significant".
Boy A responded: "I am aware."
Det Gda Doyle also put it to Boy A that the blood on his boots put him in the room where Ana's body was found.
"Were you in this room?" Det Gda Doyle asked.
"No," said Boy A.
Earlier, the court heard from John Hoade, a specialist in blood pattern and DNA analysis with Forensic Science Ireland, who said that a pair of boots worn by Boy A on the day Ana disappeared had her blood on them.
Mr Hoade also said the blood spatter on the boots indicated that "Boy A either assaulted Ana Kriegel or was in very close proximity when she was assaulted".
The court heard that on May 19, 2018, Mr Hoade was handed a pair of boots which were worn by Boy A on May 14, the day Ana disappeared.
Mr Hoade said that he examined the soles and upper parts of the boots for blood staining. There were nine separate areas of blood staining which were sampled for DNA and the DNA matched that of Ana Kriegel.
Mr Hoade identified blood staining on the right boot close to the laces and on the toe.
The blood staining on the left boot was at the inner ankle, the edge of the sole and on the upper part of the boot.
Mr Hoade said that some of the staining on the boots could be identified as blood spatter. This occurs when external force is applied to a source of liquid blood, which then falls on a surface.
The blood spatter on Boy A's boot indicates that Boy A "either assaulted Ana Kriegel or was in very close proximity when she was assaulted", Mr Hoade told the jury.
The court heard that Mr Hoade also examined a length of stick which was discovered in the room where Ana's body was found. The stick measured 92cm long, and 4cm x 3cm wide.
Mr Hoade said there was blood spatter and general staining on the piece of wood. He said the blood on the stick was tested for DNA and the DNA matched Ana Kriegel's DNA.
Mr Hoade also identified a particular pattern of blood staining known as feathering on the stick which he said was "associated with a weapon that has been swung".
He said the blood staining on the wooden stick was what he would expect if the "piece of wood was used as a weapon".
Mr Hoade said he sampled the edges of the stick to see if he could ascertain if whoever used it had left any traces behind, but the only DNA he found matched Ana's DNA.
The court heard that Mr Hoade examined a concrete block, which was also found at the crime scene. He said it was a section of a 9in concrete block.
Mr Hoade agreed with prosecutor Brendan Grehan SC that the block was "quite heavy" and a person would need "two hands" to hold it.
The court heard Mr Hoade examined the concrete block and found blood on all six surfaces of it.
Mr Hoade said he did not sample the blood from the block. This was because he understood there was "no suggestion the block was used as a weapon in the assault".
The trial continues.