dennis maher is a happily married father of two. 19 years he spent in prison for rapes he did not commit are never far from his mind. for is the realization that the victims were so mistaken. >> i was trying to reason why it happened? how could they misidentify me? the first one said i didn't have a mustache, the second one i did. but they still went on with the case. >> reporter: maher is among 321 convicted men in the u.s. who have been exonerated thanks to dna evidence. a report says that witness memories are quote forgotten, re reconstruct reconstructed, updated, and misrepresented. jennifer thompson was sure the man she sent to prison for her rape did the crime. only to learn too late that dna showed it was committed by someone else. >> i can remember thinking to myself if that's wrong, if bobby poole raped me and not ronald cotton, then maybe everything i thought was true is not true. >> reporter: police forces have recognized the fallibility of witness identification and begun to adopt changes to deal with it. the police chie