For the past month, Axios has been interviewing people trusted with the nation’s most sensitive secrets. We wanted to know, in this time of acute geopolitical stress, which global threat worried them most, and which threats they thought weren’t getting the attention they deserved.

When we asked America’s foremost intelligence experts what keeps them up at night, one response came up over and over again: the risk of a crippling cyberattack.

The big picture: Homeland Security Secretary Kirstjen Nielsen said this week that the U.S. is in "crisis mode," comparing the danger of a massive attack to a Category 5 hurricane looming on the horizon. Intelligence chiefs from the last three administrations agree, and told Axios there is no graver threat to the United States.

A well-executed cyberattack could knock out the electrical grid and shut off power to a huge swath of the country, or compromise vital government or financial data and leave us unsure what is real.

The sheer number of internet-connected devices, from cars to pacemakers, means the risks are growing by the day.

Gen. David Petraeus , former CIA director: “What worries me most is a cyber equivalent of a weapon of mass destruction falling into the hands of extremists who would, needless to say, be very difficult to deter, given their willingness to blow themselves up on the battlefield to take us with them.”

, former CIA director: “What worries me most is a cyber equivalent of a weapon of mass destruction falling into the hands of extremists who would, needless to say, be very difficult to deter, given their willingness to blow themselves up on the battlefield to take us with them.” Former CIA Director Leon Panetta says the biggest threat is "a cyberattack that could paralyze the nation," while former Homeland Security Secretary Michael Chertoff says "cyberattacks on critical infrastructure from state or state-sponsored actors are the biggest threat right now.”

Russia, China, Iran and North Korea are the top U.S. adversaries in the cyber realm, but the threat extends to non-state actors and criminal groups.

“The steady drumbeat of breaches in the headlines — each more fantastic than the next — may have numbed people, but everyone should care about the cyber threat," explains Lisa Monaco, homeland security adviser to Barack Obama. "First, we are all vulnerable. Second, it won’t take a cyber 9/11 to make this very real.”

“There will be tremendous media coverage and assigning of blame after there is a catastrophic attack on U.S. critical infrastructure that results in the loss of American lives,” says Frances Townsend, homeland security adviser to George W. Bush, “but we need to spend more time now covering what is at stake and the magnitude of the growing risk.”

"Companies in the energy, financial, and other key economic sectors need to develop the capacity to share threat information in real time, and give the government the visibility and information to take action when necessary to defend us," says Matt Olsen, former director of the National Counterterrorism Center.

The bottom line: The fact that so many intelligence experts have reached the same conclusion — and feel so strongly about it — shows how much the dangers to the United States have changed since 9/11.

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