. >>> after nearly 30 years in prison, a jury found clarence mocell not guilty of a rape he said he never committed all along. a du professor says the evidence was always there to prove his innocence. victoria sanchez spoke with the expert. >> i don't hold no grudge. the only thing is that i always wanted the truth to come out. >> reporter: this case wasn't about dna. the 1980s. and police "accidentally" threw out the evidence years ago. the professor says the case came down to a simple blood test done in 1987. >> when i looked at the evidence in this case i approach today with an open mind. what i saw really, really shocked me. >> reporter: what was so shocking to daniel son was the blood test excluded clarence because he has type b blood. and the suspect was type or. >> the one -- type o. >> t they never entertained was that maybe the reason that b-type blood was not detected in the semen was that he was not the source of the semen! they never considered that as a possibility. >> reporter: 28 years ago, the prosecution argued that the victim who also has type o blood masked the rapist's