Update: Motorola has responded, claiming the blame lies with them for the situation - they mailed a number of Nexus 6s with incorrect firmware that would cause the phone to fail to start up properly. Pre-order customers are those most likely to be affected, and those persons will have an opportunity to replace their devices. Here's the full statement:

We delivered a small number of Nexus 6 smartphones with incorrect software to AT&T customers who pre-ordered.

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