and on all the second, five weeks after lauren disappeared, stephen mcdaniel, the quiet young law schoolthen, really of a crime so awful, a dismembered victim. we stephen head seems so harmless. no criminal record. the evidence against him with circumstantial. the district attorney wasn't confident. >> i was worried that unless we had more, that this was a case where everybody knew that he did it, but nobody could prove it. >> so, time to take a harder look at the evidence. >> coming up. investigators are about to discover a certain piece of deleted video. >> what was it like to see that? >> i knew we had him. >> do they though? when dateline continues. ♪ breeze driftin' on by... ♪ if you've been playing down your copd,... ♪ it's a new dawn, it's a new day,... ♪ ...it's time to make a stand. start a new day with trelegy. ♪...and i'm feelin' good. ♪ no once-daily copd medicine... has the power to treat copd in as many ways as trelegy. with three medicines in one inhaler, trelegy helps people breathe easier and improves lung function. it also helps prevent future flare-ups. trelegy won't r