Is There a Nuclear Warfare?

Cormac McCarthy has not explicitly said, and he doesn't explain it in the book, whether or not what happened there is because of nuclear warfare. Some people speculate that it's from like a meteor strike. What matters is the result and it's that the sky turns gray. Plants stop growing. There are basically no more animals. The earth is just dead. It seems there's no ability to grow anything new. And so the people become scavengers.