Federal agents say they believe someone intentionally set the fire that led to a 2013 fertilizer plant blast that killed 15 people and devastated a small Central Texas farming town.

That revelation is leading residents to wonder: Who? And why?

Mary Sanders' son Kevin was among the 12 first responders killed when the blast tore through the West Fertilizer plant. She says a few of those thoughts went through her mind, freshening the pain just as she and her family were "just finally coping" with his death.

Law enforcement experts said a suspect's intent could be important as prosecutors decide whether to pursue arson, murder or domestic terrorism charges. Intent also could pay a role in sentencing should the government proceed with a case and win.