The zoos and breeding facilities is the generally accepted theory among herpers....but outside of that, no one cares enough to look into it so they all think it was released pets.
I'm sure some pets were released at some point, and they definitely don't help the problem. But the chances of them being the main cause for it are slim, though incalculable, because as JaGv pointed out, you can never tell who is and isn't telling the truth about releasing animals.
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