Why? It's the one question we grapple with after every mass shooting.

In some cases, we many never know. But at times, we learn what may contribute to the actions. 

In the wake of the Thousand Oaks, Calif., shooting that claimed 13 lives earlier this week, it was learned that the gunman, a Marine veteran, may have struggled with post-traumatic stress disorder.

Certainly, there are thousands of veterans who battle PTSD and do not commit mass shootings, but how do advocates ensure that all veterans are getting the help they need?

"They can walk in. They can talk to our staff," said Judy Hayman, director of the Syracuse VA Medical Center. "They can talk to our benefits staff. They can talk to our mental health professionals, but you should come in and find out the services that we have available.

"For veterans that may be dealing with mental health related issues, you may not know when you first come home. It may be months or even years later, and the research shows that veterans participating in VA care have significantly better outcomes than those that are treated in the community or those that aren't receiving care."

Hayman says that often, many servicemen and women don't consider themselves veterans, so they don't take advantage of the resources. 

"We often hear from veterans that 'I don't need the services,' " Hayman continued. " 'I want to save the spot for somebody who needs them more' ... A lot of female veterans sometimes don't see themselves as veterans, or people that maybe were not in a combat zone may not see themselves as a veteran.

"If you've served, you're a veteran. Come in and talk to us."

If Hayman's word isn't enough, take it from Edward Danquer, a Korean War veteran who was the first patient at the facility, admitted in 1953. 

"I don't know why they don't take advantage of it, especially with the cost of medical insurance nowadays and breakages getting insurance," Danquer said.