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That blood splatter served as a gruesome canvass with roc written over it. >> when the letters roc was over top of it. it scribbled over it and didn't disrupt it at all. >> here was her specialization, roc must have been written after they dried, how long after? >> i have a special machine i used to make some spatter. >> in her lab, she used animal blood to test how long for spatter to dry on a similar surface. >> when i came back and was able to apply spatter to some sections of some cardboard, i was able to get some blood and writing the word roc. i must have written it a million times over ...

After all, karen had been stabbed 16 times, an attack so ferocious, it could only be a crime of passion. now the detectives were desperate to figure out what their biggest clue of all meant, those three letters written in blood. roc. this is a creepy scene. i mean, you got this scrawled in blood message saying this is my killer. i'm now dead, you find this guy. that's what it's suggesting, isn't it? >> absolutely, it's what it's detecting. >> detectives found how those letters were connected to the victim lying beneath them. roc it turned out was a man that spelled trouble for karen in the pas...

That did to her? that's a lot to put on your shoulders. >> right. i think that really affected herself esteem. frankly, i think it had an impact on the kind of men she was attracted to. >> karen came to rely on roc to take her to doctor's appointments and injections. when she asked him to move in, her friends and family thought she was asking for trouble. >>ive think that's true. is it my place to say, well, you need to go find somebody that is going to offer you a better future? >> you can't dictate terms to your kid sister? >> no you can only fix yourself. >> as it turned out, roc wasn't a f...

Person, is that a thing? the clues pointed in so many different directions, it really was -- it was a total mystery. >> reporter: there's a concept in the law known as a dying declaration. with those three letters scrawled in karen's own blood lead to the apprehension of her killer? >> coming up, investigators dig into that three-letter mystery. who or what was roc? when dateline continues. s roc? when dateline continues. cold-eeze® can shorten your cold by 42%. it releases zinc ions that some scientists believe inhibit cold viruses from replicating. try cold-eeze® lozenges. of a lifetime. i...

>> you've got this message saying this was my killer just like you'd see in a movie. >> and the ending, that was just like a movie, too. >> i can't believe what people do to each other. hello and welcome to dateline roc, in this case those three letters would tell a story all their own. the letters scrawled in blood above her body seemed like a beacon that could lead detectives to her killer. but what exactly did they mean, and what other crucial clues were hiding in plain sight? it would take a dogged forensic specialist with a keen eye to unravel the mystery. here's dennis murphy with