. >> reporter: police are led to another student named cecelia pierce. pam's intern at the high school. their interview appears in an interview on "discovery i.d." she tells police that billy and pam were lovers. >> they were friends. then they were having an affair. >> how did you find out about the affair? >> pam told me that she was in love with >> bill flinn was a juvenile. just 16 years old. >> reporter: were you technically an adult, and he was under age. >> absolutely. >> reporter: that relationship seems predatory. >> it was totally wrong. >> we've had several bombshells in this case, but another one is about to explode. cecelia reveals that she has heard the whole story, conversations between billy flinn and pam, and that she has heard that pam wanted to get rid of greg. >> we're going to conduct an audio surveillance of one pamela smart, using a confidential informant. >> they decide to ask cecelia to get wired up, wear a taping device under her clothes. >> pretty much, they, they established that, yeah. >> why? >> like, you know, they can't th

billy flynn and pam, and that she has heard that pam wanted to get rid of gregg. >> according to cecelia pierce gonna conduct an audio surveillance of one pamela smart using a confidential informant. >> they decide to ask cecelia to get wired up, wear a taping device under her clothes. >> and she goes to visit pam at her office. >> the trap is set. will pam fall in? >> hi, how are you? >> good. >> cecelia tells pam that the cops have called her in for questioning about the murder. >> pretty much, they -- they established that yeah, you had gregg killed. >> but why, though? >> because they can't think of any other reason why bill and them would do it. >> yeah, but even if i asked somebody to kill somebody, you'd have to be [ bleep ] deranged to say, okay, i will. you know what i mean? whether someone asks you to or not. >> as far as i can see it, bill did it because he loves you. >> she doesn't break into tears. she doesn't deny it. nothing! that's very telling. >> let me tell you how out of whack i was. okay, the day before this, i had a lawyer, and he calls me up and he says, whatever you do,