Investigators zero in on DNA found in Nancy Guthrie home and question gun shop owners

Tonight, as investigators zero in on the unknown DNA found at Nancy Guthri's home, NBC News is learning more about how they're also canvasing gun stores across Tucson, trying to identify the armed suspect. Owners of two different gun shops tell us investigators came to their store with a list of names three to four pages long, including photos of potential people of interest. This is one of those stores.The owner tells us he didn't recognize or sell a gun to anyone on that list. The other owner tells us on ...