Answer:
Forensic biologist
Step-by-step explanation:
The specialist needed to find or identify the killer would be a forensic biologist. A forensic biologist uses trace samples or evidence in identifying victims and/or suspects and generally supporting criminal investigations with the aid of scientific techniques. In the case described in the question, there are other samples (aside from body fluids and fingerprints) that can be obtained from the crime scene; for example, in the case of a struggle with the killer, some of the boys could have pulled hair strands from the killer. The root of these hair-strand can be subjected to DNA analysis to identify the killer. There is also the possibility of having one of the boys scratching some skin cells from the killer; these could also be useful in DNA analysis.
The forensic biologist could also determine when and where the actual death of the boys took place, this information could be vital for the law enforcement in predicting where the killer might strike.