If the FBI and U.S. Attorney's office decide not to prosecute a case, the Secret Service, the district attorney or local police can choose to investigate, but they often have even fewer investigators than the FBI, Sizemore said.
"We don't have the resources to prosecute every $5,000 or $10,000 fraud," Slotter said.
Another reason for the lack of prosecutions and convictions is the fact that many banks are unwilling to report internal thefts to authorities, according to fraud analysts.
McNelley said banks make a calculated decision whether to report internal theft cases.
"Banks build their reputations on trust," she said.
As banks make the decision to prosecute cases, they calculate if they have sufficient proof and if the incident is significant in magnitude to incur a risk to their reputation associated with having it publicly known, she said.
Some banks employ internal investigators, according to Douglas Johnson, vice president of risk management policy at the American Bankers Association.
Ultimately, the banks' bottom line and bank customers are affected. As products are priced, the cost of fraud or internal theft factors into a bank's pricing methodology, so to some extent the cost is passed on to consumers.
"So really everyone is impacted by this crime," McNelley said.
Johnson noted that at most banks, there are capital reserves in place to account for losses that might occur throughout the year from theft or other financial crimes. Typically, the bank is able to replace the money without passing the cost onto the consumer, he said.
Fraud analysts and FBI investigators say that banks can cut down on internal theft by putting in place better controls.
"Often, when you look behind the curtain the bank's controls were ignored, everybody was going through the motions but trusted the perpetrators and in many cases gave them control over the areas they were stealing from," Sizemore said.
"There are too many serious bank frauds taking place to spend valuable resources chasing a fraud where unfortunately the bank is complicit in having no internal controls and poor management," he said.
McNelley said the problem will persist, especially in times of financial distress.
"It's an issue that's not going to go away," she said. "Internal fraud has been around since the dawn of man. It will continue to be an issue."