Here is another article, Moody. It looks like a small fire broke out and when water was sprayed on it, due to ammonia nitrate, the plant exploded.

I hope the Texas curlies are okay. Based on this article, several fire fighters and near by residents were injured.
Originally Posted by Fifi.G
So there was a fire and they responded incorrectly causing an explosion?
The news said several firefighters are missing and the explosion registered ad an earthquake.