While I think it's likely the ransom email is from the actual perpetrators of this crime I don't believe it originated as a kidnapping for ransom. I believe this started out as a different crime - robbery, sexual assault, extortion, domestic argument - and the killing (yes, I believe Ms Guthrie is deceased, unfortunately) and the "kidnapping" is an after the fact red herring.
I believe professional kidnappers would have left a ransom letter on the property. Common paper, common ink, no fingerprints, no DNA, untraceable, rather than electronic communication that WILL be traced sooner or later.
Life isn't a James Bond movie and the odds are overwhelmingly that this crime started in a garden variety way done by garden variety people who committed a rash act and a panicked series of events followed afterwards.
I would look close to home.
This is all my personal opinion.