They're treating it as a crime scene, from this morning's news:

"Authorities do not yet know what caused the fire that led to the explosion, and as such are treating the plant as a crime scene and urging people to stay out of cordoned off areas."

http://www.timescolonist.com/crews-seek-survivors-bodies-after-texas-fertilizer-plant-explosion-deat...

There was no indication the blast, which sent up a mushroom-shaped plume of smoke and left behind a crater, was anything other than an industrial accident, he said.