By Keith Morrison, Dateline Correspondent

A strange thing has happened in recent years to some of law enforcement's signature tools, those pieces of evidence which have sent countless thousands of men and women to prisons all around the world. The agent of change is, of course, that amazingly accurate marker of individuality, DNA.

DNA is now helping police solve crimes which once would have languished in a cold case room forever. But as we have also learned, occasionally to our surprise, DNA has also undone convictions once considered absolutely solid.

Coast to coast, in hundreds of cases, men and women in prison for decades, some even on death row, have been released, their innocence proven beyond any doubt by advances in DNA technology. This has forced sometimes reluctant authorities to reconsider more traditional kinds of evidence that they used to rely on to win convictions.

In turns out, for example, that eyewitness testimony is unreliable. Memory studies have revealed that our ability to recall with accuracy something from the past is worryingly bad.  One study showed that the more determined we are that a memory is accurate, the more likely it is that we are wrong! Yet such testimony has put a great many people in prison. And now DNA has freed some of them.

The ability of police (or anyone else, for that matter) to determine the truthfulness of suspects under questioning has also been shown to be quite weak.  Human beings turn out to be very good at lying and not so good at recognizing the lies of others.

One of the most remarkable developments is this: something like 25 percent of inmates freed from death row by DNA evidence confessed!  Yet DNA shows someone else committed the crime. Who in his right mind would ever confess to a murder he didn't commit?  Occasionally they confess over and over again, on tape, on video.  And lo and behold, they didn't do what they admitted to.

What about all the thousands of cases in which DNA is simply not available?  We know -- in part because of the new science -- that confessions are not always true, that 'hunches' which sometimes drive investigations can't necessarily be relied on, that lies slip by uncaught, witnesses err, and certainty is hard to come by.

Which brings us to the strange case of Barry Beach, imprisoned for a quarter century now for a murder he once confessed to.

The characters in the story could hardly be more remarkable. Beach himself has established an unusually impressive record while in prison.  His defenders at Centurion Ministries worked with tireless zeal to free him, convinced absolutely of his innocence.

His accuser was a man whose own career reached the mountain top of influence, prestige and credibility in American politics, and he was impassioned in his belief that the right man was in prison.

A very special 2-hour Dateline, 'The Killing at Poplar River,' will air on NBC Friday, April 4, at 9 p.m. ET.