In an exclusive interview with "CBS Evening News" anchor Norah O'Donnell, former Homeland Security Secretaries Jeh Johnson, Michael Chertoff and Janet Napolitano reflected on the impact of 9/11 and the greatest threats to national security 18 years later. It was the first time the three former secretaries sat down for an interview together. They met at the 9/11 Memorial in New York City.

Chertoff served under former President George W. Bush and Napolitano and Johnson served under former President Barack Obama. But they all agreed that issues like cyber security and climate change are some of the toughest problems facing the U.S. today.

9/11 Anniversary

Norah O'Donnell: It's been 18 years since 9/11. We're here at this very solemn memorial. The threat has changed dramatically. We face increasing homegrown threats. What's happened?

Johnson: The terrorist threat to our homeland has evolved significantly over the last 18 years from what I refer to as terrorist-directed attacks to smaller scale terrorist-inspired attacks. Inspired either by a terrorist group overseas or by extreme right-wing violent nationalism. In fact, that type of attack is now outpacing attacks inspired by foreign terrorist organizations.

Former Homeland Security Secretaries Michael Chertoff, Janet Napolitano and Jeh Johnson with "CBS Evening News" anchor Norah O'Donnell. CBS News

Department of Homeland Security

O'Donnell: Secretary Johnson, how do you view the agency right now?

Johnson: I have to say, I view the agency with a fair amount of despair and dismay. I think our old agency has become overly politicized. It is overwhelmed with the immigration mission, which is itself overly politicized, very, very emotional.

O'Donnell: Secretary Chertoff, the Head of Homeland Security, the Head of FEMA, all the agencies overseeing immigration have acting leaders, not confirmed. How does that hamper or hurt our national security?

Michael Chertoff: The reality is if you don't have a Senate confirmation, you're living day-to-day. And that affects not only your ability to plan strategically, but the way in which you're received by other agencies, other departments and even other countries.

Climate Change

O'Donnell: You all agree climate change is a big threat.

Johnson: Yes.

Janet Napolitano: Yes.

O'Donnell: When people say, "Well, I don't understand, how does climate change affect homeland security?" What are some real world examples?

Napolitano: When you're talking about global warming as a whole, that affects migration patterns, that affects areas of the world where the agricultural economy has been lost. And therefore, they are perhaps more ripe for the rise of terrorism, terrorist recruitment. So there are all kinds of downstream effects from global warming.

Election Security

O'Donnell: We're now 14 months away from the next presidential election. How concerned are you that foreign governments are gonna do everything they can to cause chaos?

Napolitano: Very.

Chertoff: The Russians are the most blatant. But the Chinese and Iranians play this game too.

Johnson: The election may be 14 months away but the campaign is now. And we are probably, as we speak, suffering from the foreign influence on our democracy through pushing out extremist views and fake news.