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Defense Secretary Ash Carter on Sunday said the U.S. hasn’t done enough to help veterans deal with post-traumatic stress disorder and other mental issues, following an airport shooting in which the attacker was a veteran that may have suffered from the illness.

“No — we keep learning more about how to deal with this kind of illness, we’re gonna learn more and we have to do more absolutely,” he said when asked on NBC’s "Meet the Press" Sunday morning.

Related: Brother Says Airport Shooting Suspect Esteban Santiago Sought Help for Mental Problems

Carter said PTSD — "the so-called invisible wounds of war — are something we do take seriously and we have to take seriously."

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But following Friday’s shooting — in which former soldier Esteban Santiago opened fire at the Fort Lauderdale airport, killing five and wounding six people — the issue of veterans’ mental health and medical care has again drawn national attention.

Related: Airport Shooting Suspect Charged, Faces Possible Death Penalty

Santiago served in the National Guard, and federal law enforcement officials told NBC News that he was undergoing treatment for mental health problems, complaining that he was hearing voices.

Carter also weighed in on the threats posed to the U.S. by Russia and North Korea, calling the former’s involvement in the American election an "aggressive act against our very democracy."

Carter said the Russian sanctions imposed by the Obama administration in retaliation for the election hacking "represent a beginning and not the end, a floor, not the ceiling."

He went on to say that further action will be up to the next Congress and the new administration, but didn’t rule out a military response.

“I think we should not limit ourselves, when cyber is the means used to commit aggression against us, to a cyber response. That part is right. I don't think it should be a military or purely military response," he said.

And he warned that North Korea’s nuclear weapons capabilities are a "serious threat" to the nation.

"Their nuclear weapons and ballistic missile defense programs are a serious threat to us. We try to stay ahead of that and we are staying ahead of that with our missile defenses to make sure we've upgraded their number, their type, so that we're sure we can defend ourselves," he said.

Carter said the Defense Department’s job is “to stay one step ahead of the North Koreans," and that they’re prepared to shoot down a North Korea missile launch or test "if it were coming towards our territory or the territory of our friends and allies."