Interested in Russia Investigation?Add Russia Investigation as an interest to stay up to date on the latest Russia Investigation news, video, and analysis from ABC News.
"The United States will no longer tolerate your interference. You will be exposed. And, you will pay a high price," Nielsen said at a cybersecurity summit in New York.
Nielsen also defended the U.S. intelligence community’s assessment of Russian efforts in 2016 even as President Donald Trump has been criticized for casting doubt on those conclusions -- suggesting at his Helsinki summit with Russian President Vladimir Putin that he gave equal weight to Putin's denials.
"Two years ago as you know, a foreign power launched a brazen, multi-faceted influence campaign to undermine public faith in our democratic process and to distort our presidential election. That campaign was multifaceted, it involved cyber espionage, leaks of stolen data, cyber intrusions into voter registration systems, online propaganda, and more," Nielsen said.
“Let me be clear: Our intelligence community has it right," she said. "It was the Russians. We know that. They know that. It was directed from the highest levels. And we cannot and will not allow it to happen again.”
Nielsen’s comments Tuesday came a week after she spoke about cyber-related issues at the annual Aspen Security Forum — a performance that some in the cybersecurity community worried lacked a clear message about Russia’s efforts against the United States and Putin’s role in those efforts.
Her statements Tuesday were unequivocal.
"Although no actual votes were changed in 2016, any attempt to interfere in our elections is a direct attack on our democracy, it is unacceptable, and it will not be tolerated," she said. "Mark my words: America will not tolerate this meddling.”
Nielsen was speaking at the Homeland Security National Security Summit, which brings public and private leaders together to lay out a vision for combating cyber issues. Others scheduled to speak were Vice President Mike Pence and FBI director Christopher Wray.
She warned that there is an "urgent, evolving crisis" in cyberspace and echoed some of her national security colleagues in noting that "adversaries’ capabilities online are outpacing our stove-piped defenses."
Noting that the next attack is likely to reach Americans "online rather than an airplane," she also warned that cyber threats exceed the danger of physical attacks noting that anything and anyone could be a target at any time.
Just this month it was reported that a small bank in Blacksburg, Virginia, was a target of Russian hackers who stole $2.4 million over the course of two weekends, one of which was Memorial Day weekend.
The DHS secretary implored the U.S. to heed her warnings.
"We are in crisis mode—the Category 5 hurricane has been forecast and now we must prepare," Nielsen said.