From Discover magazine’s blog:
What’s Easier to Rig—the U.S. Presidential Elections or a Slot Machine?
Steve Freeman, a visiting professor at the University of Pennsylvania, compared the vulnerabilities of the two in his book, with some pretty alarming results. Among the problems he found:
–Unpredictable voting machine software is kept secret, while gambling software must be kept on file with the state.
–State inspectors randomly inspect gambling machines to ensure their software and computer chips haven’t been tinkered with. Voting machines don’t need to be checked, and no one knows what’s in them anyways.
…
When will we have a transparent verifiable election system?
All About Voting 
Most states have source code escrow wherein the source is kept in a secure facility and released to the State (or customer jurisdiction) whenever a “trigger provision” is reached. Some states, e.g., California, require that vendors also deliver a copy of all source (plus other stuff) to them directly.