During a classified briefing in the Office of Senate Security, Homeland Security Secretary Janet Napolitano and White House counterterrorism adviser John Brennan showed lawmakers how a hacker could breach control systems of the city’s electric system and trigger a ripple effect throughout the population and private sector, according to a source familiar with the scenario.

“The fact that we could be subject to a catastrophic attack under the right circumstances and we now know some of the things that would help us to protect against such an attack, that’s why it’s important now for the Congress to take this up,” Napolitano said in an interview with POLITICO.

During the simulation, the hacker gains access to the electric supply’s control system through a simple “spearphishing” attack, in which a worker merely clicks on a link in an email that appears to be from someone they know.