Not only ATT but T-Mobile and Verizon customers too.
Primarily looks to be in the south from what heat maps I saw, kinda sorta near backbone hubs.
Could be a result of shit infrastructure/weather or an attack (which is also a result of shit infrastructure) but no real info afaik has been released.
I live in the upper midwest and we were hit by it too. Myself and a bunch of my co-workers were without service until about an hour ago. We're mostly AT&T and Verizon around here