'We have to do more': Defense Secretary Carter on PTSD in wake of airport shooting

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."

See photos from the scene of the attack:

25 PHOTOS
Shooting at Ft. Lauderdale-Hollywood International Airport
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Shooting at Ft. Lauderdale-Hollywood International Airport
FORT LAUDERDALE, FL - JANUARY 06: Police direct traffic outside of Fort Lauderdale-Hollywood International airport after a shooting took place near the baggage claim on January 6, 2017 in Fort Lauderdale, Florida. Officials are reporting that five people wear killed and eight wounded in an attack from a single gunman. (Photo by Joe Raedle/Getty Images)

Esteban Santiago, is shown in this booking photo provided by the Broward County Sheriff's Office in Fort Lauderdale, Florida, January 7, 2017.

(Courtesy Broward County Sheriff's Office/Handout via REUTERS)

Police question people who are evacuating from Fort Lauderdale-Hollywood International Airport near the tarmac after airport shooting in Florida, U.S., January 6, 2017. REUTERS/Andrew Innerarity

Esteban Santiago, the suspect in the mass shooting at Fort Lauderdale International Airport, is transported to the Broward County Main Jail by authorities on Saturday, Jan. 7, 2017.

(Jim Rassol/Sun Sentinel/TNS via Getty Images)

FORT LAUDERDALE, FL - JANUARY 06: People wait on the tarmac of Fort Lauderdale-Hollywood International airport after a shooting took place near the baggage claim on January 6, 2017 in Fort Lauderdale, Florida. Officials are reporting that five people wear killed and eight wounded in an attack from a single gunman. (Photo by Joe Raedle/Getty Images)
Law enforcement personnel search the grounds after a shoot opened fire at a baggage carousel at Fort Lauderdale-Hollywood International Airport in Fort Lauderdale, Florida, U.S., January 6, 2017. REUTERS/Zachary Fagenson
A man is helped after airport shooting at Fort Lauderdale-Hollywood International Airport in Florida, U.S., January 6, 2017. Courtesy Mike Starobinsky/Maxwillsolutions/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. NO RESALES. NO ARCHIVE.

Airport security personnel stand in front of one of the entrances to the baggage area of Terminal 2, where yesterday a shooter killed five people and wounded six others before he was taken into custody, at the Fort Lauderdale-Hollywood International airport on January 7, 2017 in Fort Lauderdale, Florida. The alleged gunman, Esteban Santiago, 26, is reported to have flown from Alaska to Florida with a gun in a checked bag, and after landing used the gun to shoot people.

(Photo by Joe Raedle/Getty Images)

Shooting victims are unloaded from an emergency vehicle and taken into Broward Health Trauma Center Friday, Jan. 6, 2017 in Fort Lauderdale, Fla. Authorities said multiple people have died after a lone suspect opened fire at the Ft. Lauderdale-Hollywood International Airport. (Taimy Alvarez/South Florida Sun-Sentinel/TNS via Getty Images)

Broward County Sheriff officers exit from the escalator that takes people from the baggage area of Terminal 2 where yesterday a shooter killed five people and wounded six others before he was taken into custody, at the Fort Lauderdale-Hollywood International airport on January 7, 2017 in Fort Lauderdale, Florida. The alleged gunman, Esteban Santiago, 26, is reported to have flown from Alaska to Florida with a gun in a checked bag, and after landing used the gun to shoot people.

(Photo by Joe Raedle/Getty Images)

A shooting victim is unloaded from an emergency vehicle and taken into Broward Health Trauma Center Friday, Jan. 6, 2017 in Fort Lauderdale, Fla. Authorities said multiple people have died after a lone suspect opened fire at the Ft. Lauderdale-Hollywood International Airport. (Taimy Alvarez/South Florida Sun-Sentinel/TNS via Getty Images)
A shooting victim is unloaded from an emergency vehicle and taken into Broward Health Trauma Center Friday, Jan. 6, 2017 in Fort Lauderdale, Fla. Authorities said multiple people have died after a lone suspect opened fire at the Ft. Lauderdale-Hollywood International Airport. (Taimy Alvarez/South Florida Sun-Sentinel/TNS via Getty Images)
A shooting victim is unloaded from an emergency vehicle and taken into Broward Health Trauma Center Friday, Jan. 6, 2017 in Fort Lauderdale, Fla. Authorities said multiple people have died after a lone suspect opened fire at the Ft. Lauderdale-Hollywood International Airport. (Taimy Alvarez/South Florida Sun-Sentinel/TNS via Getty Images)
Travelers and airport workers are evacuated out of the terminal after airport shooting at Fort Lauderdale-Hollywood International Airport in Florida, U.S., January 6, 2017. REUTERS/Andrew Innerarity
Travelers are evacuated out of the terminal and onto the tarmac after airport shooting at Fort Lauderdale-Hollywood International Airport in Florida, U.S., January 6, 2017. REUTERS/Zachary Fagenson
Law enforcement walk outside a terminal after a shooter opened fire at a baggage carousel at Fort Lauderdale-Hollywood International Airport in Fort Lauderdale, Florida, U.S., January 6, 2017. REUTERS/Zachary Fagenson
Law enforcement personnel take cover below a stairwell in a parking garage opposite a terminal after a shooter opened fire at a baggage carousel at Fort Lauderdale-Hollywood International Airport in Fort Lauderdale, Florida, U.S., January 6, 2017. REUTERS/Zachary Fagenson
FORT LAUDERDALE, FL - JANUARY 06: Police assist people seeking cover outside of Terminal 2 at Fort Lauderdale-Hollywood International airport after a shooting took place near the baggage claim on January 6, 2017 in Fort Lauderdale, Florida. Officials are reporting that five people wear killed and eight wounded in an attack from a single gunman. (Photo by Joe Raedle/Getty Images)
FORT LAUDERDALE, FL - JANUARY 06: People seek cover on the tarmac of Fort Lauderdale-Hollywood International airport after a shooting took place near the baggage claim on January 6, 2017 in Fort Lauderdale, Florida. Officials are reporting that five people wear killed and eight wounded in an attack from a single gunman. (Photo by Joe Raedle/Getty Images)
FORT LAUDERDALE, FL - JANUARY 06: People take cover outsideTerminal 2 of Fort Lauderdale-Hollywood International airport after a shooting took place near the baggage claim on January 6, 2017 in Fort Lauderdale, Florida. Officials are reporting that five people wear killed and 8 wounded in an attack from a single gunman. (Photo by Joe Raedle/Getty Images)
FORT LAUDERDALE, FL - JANUARY 06: People take cover behind cars outside of Terminal 2 of Fort Lauderdale-Hollywood International airport after a shooting took place near the baggage claim on January 6, 2017 in Fort Lauderdale, Florida. Officials are reporting that five people wear killed and 8 wounded in an attack from a single gunman. (Photo by Joe Raedle/Getty Images)
FORT LAUDERDALE, FL - JANUARY 06: People wait on the tarmac of Fort Lauderdale-Hollywood International airport after a shooting took place near the baggage claim on January 6, 2017 in Fort Lauderdale, Florida. Officials are reporting that five people wear killed and eight wounded in an attack from a single gunman. (Photo by Joe Raedle/Getty Images)

Esteban Santiago, the suspect in the mass shooting at Fort Lauderdale International Airport, is transported to the Broward County Main Jail by authorities on Saturday, Jan. 7, 2017.

(Jim Rassol/Sun Sentinel/TNS via Getty Images)

A shooting victim is unloaded from an emergency vehicle and taken into Broward Health Trauma Center Friday, Jan. 6, 2017 in Fort Lauderdale, Fla. Authorities said multiple people have died after a lone suspect opened fire at the Ft. Lauderdale-Hollywood International Airport. (Taimy Alvarez/South Florida Sun-Sentinel/TNS via Getty Images)
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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.

Read social media reactions to the attack:

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Reactions to incident at Fort Lauderdale-Hollywood International Airport
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Reactions to incident at Fort Lauderdale-Hollywood International Airport
#breaking reports of shots fired at FLL Live pics from #chopper4 @CBSMiami https://t.co/PkZBiERKo9
>@MxWllSolutions has tweeted a photo of a victim at the FLL airport. "Guy is bleeding profusely."
Yikes, look at this breaking news text I just got... I'm sure this will be all over the news soon... #FLL… https://t.co/zg8wbLGmby
Hope it's inaccurate or wrong but apparently there's a mass shooting at FLL airport, please be safe... prayers for the victims 🙏
NBC NEWS: 9 people shot, 1 dead at Fort Lauderdale-Hollywood International Airport; shooter is in custody, law enforcement sources say
.@WPLGLocal10 is working to confirm these tweets by former WH Press Secretary @AriFleischer of a shooting at… https://t.co/6JmUfkrI7b
I'm stuck on the FLL airport tarmac in terminal 2 and there's a roadblock on I-95. Apparently there was a shooting… https://t.co/vbnZRSMqz6
I'm at the Ft. Lauderdale Airport. Shots have been fired. Everyone is running.
#BREAKING Suspect in custody after shooting at Ft. Lauderdale Airport https://t.co/XjP9GAV2WU https://t.co/XVCWPP2GvX
Tragic news out of Florida today, with news of an airport shooting. We're 6 days into 2017 and this is our 6th ma… https://t.co/PnqUMkRLKQ
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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."

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