Why does everyone assume that Red John has a tattoo?

Seriously, it's ridiculous reading the theories saying that the list is now down to three names based on the fact that Red John apparently killed the private investigator and showed her his tattoo, and then let her live long enough to tell Pj about it.

WHY would that have been Red John killing her?
Do you have any other proof other than Pj saying "it was Red John"?
It could just as well have been the TT conspiracy that bugged their office, and later killed her.
It was so sloppy and SHE ALMOST SURVIVED, so it was for sure not Red John.

If it was I'm gonna send a letter to Heller telling him how this series turned to shit.