Hackers recently accessed the critical infrastructure of three unnamed cities by compromising their SCADA (supervisory control and data acquisition) systems, the deputy assistant director of the FBI’s Cyber Division said today.

Speaking at the Flemings Cyber Security conference in London, Michael Welch said the hackers could theoretically have dumped sewage into a lake or shut off the power to a shopping mall.

"We just had a circumstance where we had three cities, one of them a major city within the US, where you had several hackers that had made their way into SCADA systems within the city," Welch said.

The attack "was sort of a tease to law enforcement and the local city administration, saying ‘I’m here, what are you going to do about it," he said. "Essentially it was an ego trip for the hacker because he had control of that city’s systems and he could dump raw sewage into the lake, he could shut down the power plant at the mall – a wide array of things."

Welch would not clarify whether the attacks in question realated to a reported SCADA attack on a water facility in Springfield, Illinois. On Wednesday, the Department of Homeland Security denied that there was any hacking involved in the failure of a water pump at the Springfield facility.

Cyber security is "a huge growth factor" for the FBI, says Welch. He expects the bureau’s Cyber Division to double in size during the next 12 to 18 months.

"A big part of what we do is private sector liaison," he said. "At no time in our history have we had to stretch the definition of what constitutes crime more than we do now."