Caveat: The information on the Illinois Water system cyber intrusion is directly from the Illinois Statewide Terrorism and IntelligenceCenter. In fact, the title of the document is "Public Water District Cyber Intrusion". I have simply provided their explanation.
DHS's statement on theIllinoiswater SCADA Intrusion is: "After detailed analysis of all available data, ICS-CERTand the FBI found no evidence of a cyber intrusion into the SCADA system of the Curran-Gardner Public Water District inSpringfield,Illinois." Conversely, DHS and the FBI found no evidence a cyber intrusion did not occur.
Currently, there is a lack of CONTROL SYSTEM cyber security forensics and logging. DHS needs to mandate the implementation of appropriate control system cyber forensics and logging or the Curran-Garner scenario will continue to be repeated. Until that time, it really is an engineering exercise to understand why control systems and plant equipment operate or fail in unexpected ways.