The e-voting systems in use throughout the US are proprietary and are not reviewed by independent security analysts. We have to take the vendors' word that the information is secure. As dramatically illustrated in HBO's expose, that is emphatically not the case.
The most widely used e-voting systems are provided by Diebold. First the team went into the Diebold General Elections Management System (GEMS) and altered results. Diebold said this was not possible without creating an audit trail, but nothing the team did produced an access log. They were able to alter results in a number of ways. Speaking of access, votes are stored in a Microsoft Access database. I am very familiar with Access' security, but I also know from personal experience that it can be circumvented and as shown in this documentary, it doesn't take a knowledgeable person very long to do it.
As a final test, the team successfully altered the results of a mock election by directly altering the memory card used in the voting machine. According to Diebold it is not possible. The security analyst only touched the memory card for the voting machine prior to it being inserted in the machine. He never touched the machine itself, nor did he have access to the card after it was put into the machine. When the voting machine came up it printed a 0 total report, showing that there were no votes already in the unit. 8 total votes were cast, 6 were "No", 2 were "Yes", then a final tally was printed. It showed 7 "Yes" and 1 "No".
To verify these were actually recorded results and not just the result of altering the report, they loaded the results into GEMS. It showed the altered results. According to Diebold this is absolutely not possible. In reality, it absolutely is. This is the most likely way that John Kerry ended up with negative votes in some Florida counties in 2004.