Theories
Heather

This Section Will Contain Spoilers!

JWJ's letter sounds odd with the way he ends their relationship:

    • "She was a lovely young woman and I admired her all the way to the end of our arrangement."

Heather's letter talks about her unhappiness with her boyfriend, Allan, and that she will pay him a visit when his parents are gone and mentions in her letter to JWJ about a stereo and a DJ that is her favourite.

    • Man is found dead of large puncture wound to liver in Texas (green dot on map) while parents are gone to a gala in Dallas with stereo blaring (Body #1, possibly Allen?).

She decides to go see the only man that "gets her" as per her words in the Blue Letter to JWJ. So she travels from there to Darlington. On the way she either has a thirst now for killing or has a score to settle with another man and kills again.

    • Man found dead of penetrating abdominal trauma to liver in Meeman-Shelby Forest State Park in Tennessee (B on map) (Body #2).

She then continues on towards Darlington and decides to kill again this time in Indian Head, Maryland.

    • Man found washed up dead on Potomac River in Indian Head, Maryland (C on map) (Body #3).

She then continues on to Maryland and meets JWJ at the lighthouse at Concord Point Lighthouse.

  • JWJ at this time has more privileges such as writing to and from pen pals and possibly even a day pass from the facility

If so he meets her at the Lighthouse with his day pass and after she admits to him what she has done he is disgusted and kills her.

    • Female found dead by tourists at mouth of Susquehanna River (Checkered flag on map), which is where the Concord Point Lighthouse sits.

JWJ then loses all return pen pal privileges and is kept on a much stricter lock and key program until we enter the picture many years later.

This also explains his strangely worded ending to their relationship of "I admired her all the way to the end of our arrangement."