Over the years there have been several different case officers assigned to the Lyon Case. If you were to ask each one candidly who they thought was the best possible suspect, you would probably get many different answers. Some might think that one or another potential suspect or person of interest discussed here was involved, while others might have an entirely different person pegged for it.
In reality, probably NONE of those officers would offer their own theory, but would go with the official answer for the last 34 years - that no one was ever positively identified or charged.
All that aside, I really do not think that there is one such a person who could be described by police as "the suspect".
The closest that I have seen them come was in March of 1987, when officer Caswell stated in a press conference that Fred Howard Coffey, Jr. seemed to be the strongest lead in the case since it began. He stated this in various ways three separate times in the same interview.
In that interview he stated that police DID NOT think that Coffey was the Tape Recorder Man, but he did not state why they thought this.
After two weeks of intensive investigation, police announced that they could not tie Coffey to the disappearance of the Lyon girls.
Police did look into Coffey very closely and while they could not definitely link him to the girls disappearance, they also did not in any way prove that he did NOT abduct them. They simply could not state positively that he was at Wheaton Plaza or vicinity on 25 March 1975. Whether or not they ever interviewed Coffey in North Carolina is questionable. I do not think that they did.
In the minds of some Montgomery County Policemen (past and present) Coffey may be "the suspect", but that was not the impression that I got when speaking with the current case officers. They seem to be open to any and all possible solutions.
As to forensic evidence, Police have no bodies in this case, no blood, no dropped items, no bits of clothing, jewelry, etc. At least if they do, they have never stated such.
They have reports that dogs picked up the girls' scents along the route between Plaza and home, but could not state definitely that it was going or coming, or what day the scent was left. But then, we know that they walked through that area to get to the Plaza that day, so of course their scent should have been there.
There was no communication from the abductor or from the girls after they disappeared. There was an extortion attempt, but police decided that it was a hoax separate from any abduction.
They have eyewitness testimony about the Tape Recorder Man, but that same witness stated that they went their separate ways, and no eyewitness ever stated that he or she saw the girls getting into a vehicle of any kind - either voluntarily or involuntarily.
There has been some eyewitness testimony (mostly secondhand) about persons suspected of having abucted the girls. But evidently none which caused any arrests.
The naming of a suspect is a specific legal action. The reasons that I stated above are my opinion as to why police have not named one. They are not inside knowledge of police or prosecutor decisions in this case.