"I don't believe you're going to find any biological relationship between the (shooter) and the victims, " Blair County District Attorney Richard Consiglio said, adding the three victims "were not immediate family" of one another.

CNN: 4 dead, including shooter, in central Pennsylvania

"We're finding that when you rub a natural biological surface that has an orientation - that has a grain, if you will - and you run that grain perpendicularly to the long axis of the ridges, we find a dramatic increase in friction, " Dr Dominy told BBC News.

BBC: Artificial finger tests evolutionary origin of prints

We're looking here, potentially, at DNA-based biological identification.

NPR: Pioneering DNA Forensics

So far, that re-examination has uncovered 26 cases where the technician allegedly failed to detect biological evidence critical to the cases.

CNN: More than 800 NYC rape cases reviewed for mistakes

"One of the problems with existing electronic nose instruments is that they're very large and another thing is that they have limited similarities with the biological nose, " Dr Pearce told the BBC.

BBC: Scientists develop 'nose-on-a-chip'