A new article offers some clarifications about the "Plant" involved and what materials were there, but doesn't give much new information about how the explosion might have occured.

A fairly good (not too technical) description of the differences between ammonium nitrate that they now say possibly/likely was present in some quantity at the site and the anhydrous ammonia that was originally assumed to have been the only thing there.

The facility in West, Texas was not a manufacturing plant as some previously thought, but was primarily a distribution center where farmers could pick up fertilizers. The company itself had very few employees, although there's still no good report of how many workers were present and/or how many others nearby were hurt.

See what they say at Texas fertilizer plant also stored explosive chemical used in Oklahoma City bomb

(The headline is a little more sensationalist than necessary, but the article isn't bad.)

John