Four weeks after a fertilizer plant exploded in Texas, investigators have yet to announce what prompted the fire and touched off the massive explosion that killed 14 people.
They are especially interested in the volunteer fire crews in the town of West, who may not have been fully aware of the dangers they faced.
Associated Press interviews with first-responders suggest that firefighters' foremost fear was a poisonous cloud of anhydrous ammonia. But the greater threat turned out to be the vast stockpile of ammonium nitrate, which can also serve as a cheap alternative to dynamite.
State and federal agents plan to release some of their findings Thursday. Last week, the Texas Rangers and county sheriff's office launched their own criminal investigation, joining a separate federal and state probe.
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