bruce ivins. >> bruce ivins, he was the guy that controlled that flask. it was in his possession, and that told the investigators that maybe bruce ivins was the guy. >> narrator: the fbi discovered ivins had spent a lot of time alone at night in the lab. >> his lab hours spiked in august and then, really, september and october of 2001. never before and never after had he shown anything like that-- those late night and weekend lab access hours. that was extraordinary. >> narrator: they questioned ivins. >> ultimately, at the end of the day, he had several different stories. for instance, one of them was, "well, i just liked to go in there to get away from a bad situation at home anbe left alone." >> ultimately, i think in one of the interviews, he conceded, "i just... i don't have a good reason." >> narrator: ivins' colleagues got wind of the fbi's suspicions. ivins' boss asked him about it. >> i said, "bruce, you know, they say that you were keeping odd hours." i said, "i know you work a lot of, you know, odd hours, all of us do, you know. but do you reme