The unrelenting hail of gunfire that erupted on the Las Vegas Strip Sunday night assailed many more than the hundreds of people who were wounded or killed in the chaotic frenzy. For thousands of others who witnessed the unimaginable carnage firsthand but were not physically wounded, the shooting assailed their very hearts and minds.

Many will endure weeks or months of nightmares, anxiety and hypervigilance. A smaller group will develop psychiatric illnesses that could require professional help and linger for years or even decades. But all of them will be changed forever.

I’ve seen it unfold repeatedly in my three decades of researching the mental health effects in survivors of the nation’s most infamous disasters, from the Oklahoma City bombing to the Sept. 11, 2001, World Trade Center attacks in New York City.

Some effects are remarkable. For instance, some witnesses of the WTC attacks felt guilty for surviving while their colleagues had died. Others became claustrophobic and avoided tunnels or subways. One person reported having flashbacks during bright mornings like the one on Sept. 11: “Not a cloud in the sky and I look up: ‘Oh, my God, it’s that kind of a day.’ Those bring it back. … Gorgeous days bother me.”

Based on my research, the good news for those exposed to last weekend’s violence is that while many of these symptoms can be severe in the short term, a majority of people are resilient and do not develop psychiatric illness, even after the most extreme exposures.

The most common psychiatric disorder to develop after most disasters is post-traumatic stress disorder (PTSD), which can occur in up to about one-third of survivors of severe disasters. Post-traumatic stress symptoms include intrusive memories, dreams, flashbacks and hyperarousal symptoms such as hypervigilance, jumpiness and sleep disturbances.

Most of these symptoms resolve over weeks to months, but people who develop PTSD may suffer from these symptoms for years or decades.

There are well-established treatments for this disorder, including various medications and psychotherapies that are effective for PTSD arising from exposure to virtually any type of trauma. However, it can sometimes be difficult to determine if someone needs treatment because post-traumatic stress symptoms and distress are nearly ubiquitous. Even survivors of the Las Vegas shooting who do not develop mental illness will probably be profoundly affected and bear the scars of invisible psychological wounds. For these survivors, as for the physically wounded, images of the event may be emblazoned in their memories for a long time to come.

During such extreme and terrifying events, time moves slowly, stretching the danger into a long-endured ordeal. The adrenaline response is intense. It may take many days for jitteriness to begin to subside. People may feel unsafe, especially in open areas, and may want to avoid outside spaces. Hypervigilance may continue for a long time.

In an interview two months after the 1991 mass shooting at a Luby’s cafeteria in Killeen, Tex., one survivor stated, “I can’t sit in a restaurant with my back to the wall since then. I have to be facing the door, able to see everything in the room.” Another survivor reported that when someone enters the room, his eyes fixate on the person’s hands, involuntarily scanning for a weapon.

Some people may develop a changed perspective of the world, feeling that it is an unsafe and dark place. Individuals may even view themselves more negatively, perhaps insufficiently heroic.

A flight attendant still reeling from the WTC attack blamed herself for giving her flight shifts to a colleague so she could have time to recover from the newly realized dangers of her profession. The colleague died in a plane crash, two months after 9/11. “I felt such terrible guilt. … She had the guts … to go back to work, and I was too scared,” the attendant reported.

These types of reactions can persist for many weeks, months, or even years. It is important to underscore that most people with these responses do not develop psychiatric illness. For most people, the responses gradually fade; however, the memories may never go away.

People may be helped through crisis counseling such as “psychological first aid,” which includes providing compassionate gestures, connecting people with their loved ones, and offering resources such as education and other practical support.

For those who cope successfully, traumatic events sometimes bring positive change to their lives. They may find new meaning or feel gratitude for things they had not appreciated. They may gain new perspectives on life and suddenly begin to prioritize what is really important to them.

But for those individuals who have negative symptoms or distress that worsen over time, I recommend consulting with a mental health professional who can help determine whether they have developed a psychiatric disorder and point them to the best types of interventions.

From watching the footage of the Las Vegas shooting, it is clear from my research experience that survivors will collectively endure a full spectrum of mental health effects in the short and long term — whether they were physically struck by gunfire or not.

It was, after all, the worst mass shooting in modern U.S. history, a horrific tragedy in which thousands of people feared for life and limb and witnessed friends shot down in cold blood.

To help these survivors, the best we can do is to offer compassion, allow them space but be present as they need. Listen thoughtfully if they want to talk about their experience and feelings. Help them with specific needs, and be patient as they process their ordeal and seek to restore order to their lives.

For most survivors, that should be enough.

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