The leader of the gang (David Ghantt), was Loomis Fargo's vault supervisor and he didn't show up for work the next day...he disappeared. Thus he became their first suspect. He decided that he would take off for Cozumel, Mexico with $50,000 (anything more than this amount and authorities are notified), and that the second member of the gang (Steve Chambers), would send him large sums of money monthly to live on for a year or two until the heat died down. Instead, his buddy Steve, hired a hitman to take Ghantt out so he could keep the money. Why do criminals trust other criminals? The FBI ended up saving Ghantt as they found him through tracing a phone call and arrived before the hitman did. As they say (whoever they are), there's no honor among thieves.
Instead of being smart with the money, Steve Chambers' wife went to a bank and asked what's the most she could deposit to where the Feds wouldn't be notified. Just her saying it got the Feds very interested and they started investigating she and her husband. Come to find out they were somehow able to move from a single-wide trailer they lived in to a mansion in a ritzy neighborhood. The alarm bells were ringing loudly. The FBI got a warrant to look in their home and they found duffel bags filled with money; they actually recovered 95% of the money stolen.
Another thing the FBI thought was funny about these amateurs is that the gang didn't know how much space it would take to steal the amount of money they did and they didn't have enough room in the van they used so they ended up leaving a couple of millions of dollars in the armored truck. Anyhow, you can see why they call it the "Hillbilly Heist."