My understanding is that the FBI has never ruled Mel Wilson out. Vicki can correct me if I am wrong.
As far as I know he has not been investigated. Only briefly looked at.
Originally, I spoke with SA Curtis Eng in January 2011. He called after Jerry Thomas spoke with him about my father. Eng's first introduced himself. He told me had spoke with Jerry Thomas then....with a sigh/breath asked "So...Why do YOU think YOUR dad was Dan Cooper?"
We spoke for a few minutes. I gave Eng my father's FBI #144034A. I could hear him typing it into the computer. He hemmed and hawed for a few seconds, then asked me to email him a short followup of our conversation and he would contact me with further questions.
About three weeks went by and I did not hear from him. I called JT and, in return he called Eng. Jerry related to me what Eng told him. Eng did not pursue looking into it further as he thought my dad did not look like the sketch.
So...I do not know the extent of anything the FBI did ...or did not do. I do know that I have never been contacted telling me they ruled him out on DNA or fingerprint comparison.