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The sickening thuds of bodies hitting the concrete, the sound of bullets whistling by, the blood — these are the memories from the Las Vegas mass shooting that Glen Simpson fears will be forever with him.

Simpson, an advanced emergency medical technician for Community Ambulance, was one of the 16 EMTs working alongside off-duty firefighters at the Oct. 1 country music festival when the shooting began.

For Simpson and many of the other first responders who rushed to the horrific scene, the nightmare is far from over.

"When I close my eyes, I'm paranoid," Simpson told NBC News shortly after the Oct. 1 shooting. "It's been difficult to sleep at night. And on top of processing what my team's been going through, I have to process that one of my friends that I'd been texting the entire weekend was among the dead."

For the doctors, nurses, firefighters, EMTs and bystanders who witnessed the deadliest mass shooting in modern American history, the road to emotional recovery is just beginning.

Most will adapt. Some will recover. But for 20 percent of firefighters and paramedics on the scene in Las Vegas, full recovery from post-traumatic stress disorder (PTSD), an illness in which witnesses or victims of a traumatic experience relive the tragedy, is not guaranteed.

'Crying isn't a weakness'

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Traumatic events can injure the brain, but unlike bullets that cause a physical wound, harrowing experiences can cause flashbacks, paranoia, and frightening dreams that affect relationships and work. Symptoms of PTSD — nightmares, insomnia, memory problems or feelings of isolation — don’t always show themselves immediately and, in extreme cases, can lead to suicide.

Researchers estimate that 28 percent of mass shooting survivors will develop PTSD. Seven to 19 percent of police officers experience PTSD, well above the 7.8 percent national average, according to the Veteran Affairs Bureau.

Related: How Virtual Reality is Helping Soldiers with PTSD

On the night the carnage unfolded, the International Association of Firefighters had dozens of off-duty members at the music festival — 12 were shot while helping others in the audience and two were hit while performing CPR.

"When you run a call like the fire department had [during the Las Vegas shooting], it could be the straw that broke the camel's back," when it comes to emotional trauma, said counselor Ray Rahne, the IAFF's 9th district vice president.

The IAFF, which also provided mental health support to its members after the Orlando night club massacre and Hurricane Harvey, has a track record for providing swift care for its members. In Las Vegas, the organization's support group has been going from firehouse to firehouse, encouraging first responders to talk about what they've witnessed — an important first step to preventing PTSD.

"Crying isn’t a weakness. We all have emotions and we need to let those out."

Rahne was diagnosed with PSTD after responding to the 1999 mass shooting at Columbine High School. But he prefers to call it PSTI, because the term ‘injury’ suggests the ‘disorder’ is curable.

Two weeks after the shooting, EMT Simpson says he's doing better. His team received crisis management help from psychologists on staff, although a few members are still struggling.

"Employees are going through a range of emotions, including anger and fear," said Simpson. "We've had some employees back out from events with large crowds."

Related: PTSD By the Numbers: Hundreds of Thousands of Soldiers Affected

Recovery from a traumatic event is "not about being macho," said Rahne.

"Crying isn’t a weakness. We all have emotions and we need to let those out. It's OK to get professional help," he said. "You can be cured and go back to being an effective firefighter.”