FBI special agent Alice Redmond
After the first murder, Cupertino police detectives assumed that the killer had to live close to the victim because the video seemed to show her inside some kind of basement, which they figured that must be his house. This belief was confirmed after the next videotapes showed the exact same interior and all the subsequent victims were in the Bay Area at the time of their disappearances. But that's a lot of people and didn't really tell the police very much because some of the victims were as far away as San Francisco when they disappeared. They guessed the first victim was probably close to the perps house when she disappeared and they were right about that but still weren't able to find him, they pulled everyone with violent crimes on record in the zip codes and everyone had an alibi.
The police believed that the killer had to be a single male until the fourth tape with the female voice on it and lost a lot of time because of this. At that point the FBI got involved.
The voices of the two people heard on the fourth tape provided little material for the authorities to research, since they were mostly grunting and moaning somewhere offscreen while the victim expired (TIFFANY you need to cut this part out you gross psycho.) It is still unknown how much Sally's involvement continued outside of this murder, as her voice is only seldom heard on other recordings. She insisted that she was only involved in that one killing, but her mid-trial suicide seems to say otherwise.
Unlike Ted Bundy, the Balloon Mask Killer always killed the same way. Everything was always very planned for him and he took advantage of his image as a handsome family man to create situations where he was perfectly able to enact his fantasy. No one ever escaped. After eight more practically identical videos the FBI was no closer to identifying the man who escaped every assumption they had about serial killers,.... until a young smart agent graduated from Quantico...
Special Agent Alice Redmond graduated at top of her class from Berkeley with a bachelors in Mathematics and specialty in statistics. She got recruited by the FBI right out of college in 1986 and got put in the case right away. She had a totally different idea about how to find the killer. She was going to use math to find out who he was by analyzing messed up posts on the internet by sex freaks. Nobody had ever even thought about using the internet to look for evidence before but now its a common thing. All the evidence pointed toward a middle class criminal homeowner in the Silicon Valley area- the average UseNet user in the 1980s.
Alice used a method called "Pointwise Mutual Information" to analyze millions of posts pulled off of usenet by a court order. She wrote the software to do it all by herself! This let her calculate a special number that showed how often pairs of certain special words like "balloon" and "choke" and "breathe" would appear in the same posts together on average anywhere on the usenet. Then she looked at every usenet group individually to find which ones were above average. It turned out that the PMI for the words she chose had doubled for alt.sex.stories. And then when she narrowed the down to analyze individual users, she found an account that was also active in talk.social.bayarea.
The math Alice did was complicated but the FBI website has some formulas for it:
The program counted every time a chosen word1 would appear in the same post as another chosen word2. Then it would divide that number by the total number of posts overall (a number called N). That's the probability that if you picked a random post, it'd have both words in it, P(Word1,Word2). Then they'd do the same thing for the individual words to get P(Word1) and P(Word2).
Then they'd put all of those in the formula! The PMI for "rubber" and "choke" was 0.3 on average, but 0.7 in talk.social.bayarea! He nearly doubled the PMI with his hundreds of posts. It was 6.5 in Edmund's posts!
Alice said she got the idea for doing this type of randomness analysis on text from her favorite story, "Pierre Menard, Author of the Quixote" by an author named Jorge Borges.