the csi of r.o.c. >> i remember thinking, wow, i wonder if that's what wrote these letters. >> were thosehree little letters really what they seemed? when "written in blood" continues. >>> the handwriting was on the wall. and forensic specialist anna cox was intent on breaking down the key piece of evidence implicating karen's ex-boyfriend roc. those three letters in blood, r-o-c. >> you would spend hours looking at these letters. >> i did. >> you actually cut the sheetrock out of the place and took it into your lab. >> yes. >> i had to know everything about those letters. >> using a high-powered microscope, she did an analysis of the blood spatter. the flung spatter served as a gruesome canvas for the letters r-o-c written over it. >> when the letters r-o-c were written on top of it it skimmed right over it and didn't disrupt it at all. >> here was her central observation. since the specks of blood weren't smeared, that means r-o-c must have been written after they dried. but how long after? >> i have a special machine that i use to make some spatter. >> in her lab, she used animal blood