The detective who led the investigation into the horrific attack on two young boys by two brothers said the pair showed no remorse when they were interviewed.
Detective Superintendent Mick Mason said the traumatic nature of the incident affected many of the officers who worked on the case.
He said: "This is clearly the most traumatic investigation I have ever come across. Not the most difficult - but the most traumatic."
Asked about how the brothers appeared to his interviewing officers, Mr Mason said: "I'm aware that both boys gave an account - a quite clear account of their versions of what took place on that day.
"There were not particularly shaken, I don't think, by the events, which took us by surprise.
"We expected a lot more trauma from the two young offenders."
The senior detective added: "I think they understood what they'd done. Whether the gravity had sunk in then, I don't know. Whether it's sunk in to them since, I don't know.
"From what I'm told, they didn't show any remorse during interview."
Mr Mason said he did not know whether the pair had shown any remorse since the police interviews.
He said: "You think with the severity of the attack it must have sunk in and they must be realising what they have done. I'm not sure about that."
The officer said the two victims were recovering well but had been through a terrible ordeal.
He said: "Psychologically, this has got to have an impact on you.
"They've received a lot of support in relation to getting back to some normality.
"You can't go through an incident like this, so traumatic, and not be affected."
He said the South Yorkshire Police team had also been affected by the investigation.
Mr Mason said: "It was a very difficult case to deal with.
"I've got children and when you see what happened on that day, you get briefed and you actually go to the scene and see the young boys and meet them then, yes, it does have an effect on everybody."