I want to explain a few things in light of people thinking IF must have professionally cleaned up or LE would have arrested him on the spot because to do otherwise would be to leave a dangerous killer in the home.
First, the police may know immediately that a person likely did it and could pose a danger. However, without probable cause to issue an arrest warrant, they cannot arrest them, no matter how dangerous that person may seem to them. And in this case, that could mean that the boy explained why he was covered in blood and that they needed a blood spatter expert to analyze clothes, for example. That could also mean that there was no, visible blood and the boy said he did not go near the body, but then they found a spot or two on clothes discarded in his room, or on his person, and sent it to be tested to see if it was Leila's. And BTW, it is possible for someone to stab another to death and not have more than a few drops of blood on them. It just is.
Second, even with probable cause, the district attorney may not authorize and arrest right away for fear the case lacks sufficient evidence for a conviction. (I think that has happened in Kyron Horman's case). Thus, in this case, the independent witness making a contradictory statement may have put a wrench in the plan to arrest, especially without overwhelming physical evidence or a confession. That's because the speedy trial right clock begins ticking right after an arrest.
Oh and those who claim he must have cuts on his hands due to stabbing someone to death are just wrong. That can happen and but it is not a requirement. Take the case of 13 year old Billy Keenan who stabbed two neighbor girls to death. He had no cuts, (although he did have some blood spatter on his jeans - not huge piles of blood all over himself as some think is necessary, BTW, just a bit of blood spatter on his jeans).
Loss of Innocence: A True Story of Juvenile Murder: Eric J. Adams: 9780380759873: Amazon.com: Books
I think that in this case there was blood evidence but they needed an expert to analyze it. I think that the neighbor's statement caused a huge delay in an arrest. I think the lack of a confession right away also slowed things down.
I do not assume this kid was a cool and calculating murderer with sophistication beyond his years who easily hid evidence and hid his involvement. I think there was enough there that they knew he was the one but not enough to arrest and/or for the prosecutor to authorize an arrest. I also think he has now confessed.