We can only take this with a grain of salt, but a commenter on DownDetector, as spotted by @TheFallGall, claims that they are a “tech rep” at Verizon.
“It has been confirmed that the issue was caused by an unauthorized shut down of the electrical grid specifically targeted at the cell towers all across the US, It seems to have been done by slowly and systematically shutting down these lines, sort of like a spider building its web. At first, it was just local, then state, region, and now nationwide. Affecting not only Verizon, but other carriers as well, to a smaller capacity as they were just subs on the main Verizon towers.”
Once again, this is statement is unconfirmed and Verizon has not mentioned the cause behind the major outage.
