The West Texas fertilizer plantexplosion is now under criminal investigation.
Fourteen people were killed last monthwhen the facility caught fire and exploded. The state Fire Marshal's officesaid it has ruled out four possible causes.
The explosion registered as amagnitude 2.1 earthquake and could be felt 50 miles away.
Investigators have interviewed morethan 400 people trying to figure out exactly where the fire started and itscause.