Paige Melanson, wounded in the shooting, hugs dance student Baylee McConaghy, 11, as Pastor Robyn Garcia looks on. Garcia spent hours at Las Vegas’s convention center after the shooting, talking with people whose relatives or friends had died or were missing. (Matt McClain/The Washington Post)

When she was under fire, dodging bullets at the Route 91 Harvest festival on Sunday, Megan Greene felt an odd sense of purpose. “If you’re still breathing, you’re fine,” she told a panicky woman trying to escape with her mother, who uses a wheelchair.

Greene remained calm on the long drive home to Simi Valley, Calif. But when she finally pulled into her driveway Monday morning, she burst into tears — the first outward sign of the invisible wounds that have come to plague her since the massacre.

“The simple things that used to be so easy are, just — I never want to be alone and I never want to go anywhere alone,” said Greene, 19, adding that she cannot stand the dark, constantly checks around corners and, during a rare stretch of sleep this week, scratched her legs raw.

An estimated 22,000 people were in attendance when bullets began raining down on the music festival from the 32nd floor of the nearby Mandalay Bay Resort and Casino. Fifty-eight people died and about 500 were treated for physical injuries. Many of the rest are now grappling with the psychological scars of the deadliest mass shooting in modern U.S. history. Law enforcement officials, medical professionals, family and friends also may be affected by witnessing carnage that one surgeon described as "exactly like what we have seen in a war zone."

“We all go about our day-to-day business assuming we’re generally safe,” said Gerard Lawson, president of the American Counseling Association. “When something like this happens, it shatters your sense of safety, and that’s where the trauma comes in.”

Offering treatment to all of those who are suffering is complicated by Nevada’s lack of resources.

Nevada ranks 51st among states and the District of Columbia in mental health resources and access to treatment, according to the most recent annual report from Mental Health America, a community-based nonprofit group. Sixteen of Nevada's 17 counties are listed by the federal government as areas with a shortage of mental health professionals.

Mandalay Bay has made crisis counselors available for guests and employees, and the Clark County Family Assistance Center has offered counseling to families and victims. Religious organizations and counselors from throughout the city and region also are offering services.

“We’re trying to get our head around what we’re going to see in the weeks and months and maybe years to follow,” said Michelle Paul, a clinical psychologist and director of a mental health clinic at the University of Nevada at Las Vegas.

Paul and others said that everyone deals with trauma in different ways and there is no normal response. But it is not unusual for people who survive a mass shooting to feel guilt, have difficulty sleeping and be startled by loud noises because of their exposure to gunshots.

Greene said she is feeling all of those things. When she finally fell asleep after being awake for 43 hours, she woke up screaming. She now sleeps in her mother’s bed and went into a panic when her mother got up one morning and turned on the shower.

“Every time I close my eyes I hear the screaming of people or the footsteps, or I’m dreaming that I’m running and I’m just running and running and running,” she said.

On Wednesday, Greene participated in a weekly ritual: She went to college night at a local country line dancing bar. Many others also survived the shooting at Route 91 Harvest. There were hugs and the grateful feeling of being around people who are like family. But the night was difficult: The music and strobe lights made Greene panic. When dancers clapped during the “Cha-Cha Slide,” she ran into a corner, curled into a ball and cried.

Greene struggles with the guilt of surviving and not going back into the venue to help people. She saw a therapist Thursday who gave her Benadryl for the scratches and Xanax for anxiety. The therapist assured her that she is having a normal response to an acute traumatic event. But the days are hard. She went to lunch with a friend and did not want to walk to the bathroom alone.

“I’ve gone through this thing and no one knows what’s going on in my head right now and people are so unfazed because they weren’t involved in it,” she said. “I really feel like I’m moving a lot slower than the rest of the world right now.”

Anthony Dalrymple was at the festival with his wife, family members and friends who are off-duty police officers. The 44-year-old Army veteran knew quickly that he was hearing sustained rounds of fire from a modified semiautomatic weapon. He and his group escaped to his brother-in-law’s house, picking up strangers along the way.

“I’m doing okay. I’m handling it well,” he said. His wife is not. She has been unable to sleep and is afraid of loud sounds.

“Everybody handles it a little bit differently.” he said. “I told her she needs to let her emotions happen.”

Robyn Garcia, a pastor at International Church of Las Vegas, spent hours at the city’s convention center talking with the people whose relatives or friends were missing or had died.

Some people, she said, watched a spouse or friend die and were then told they had to leave the scene. They cried in her arms, saying they just wanted to retrieve the belongings of the dead and return to their home towns. Others had gone hours without hearing from friends or relatives who attended the concert and were prepared for the worst.

“It was very, very traumatic,” she said.

One woman lost her sister in the shooting and went into shock and denial.

“She said, ‘I put out my sister’s clothes. I’m waiting for her to come home and then we’re going to see our mom,’ ” Garcia recalled.

The next day, the woman asked Garcia for prayers. And a day later, she told Garcia she was having a hard time but trying to stay strong for her sister’s daughter.

Emanuel Maidenberg, a clinical professor of psychiatry at UCLA, said post-traumatic reaction can last from four to eight weeks, though circumstances differ from person to person. Paul said the vast majority of people affected by an event like the mass shooting will have their trauma run its course and can soothe themselves through self-care, self-compassion and community connection. Others, however, may develop and be diagnosed with post-traumatic stress disorder, major depression or panic attacks.

A traumatic event like a mass shooting can also take a toll on those who are there to help. Tirrsa Isom, 35, spent much of the week speaking with families at the convention center. Many of them, she said, simply needed someone close by and, if they were ready, someone to talk to. Isom helped relatives of the victims process their losses, watching them realize that their loved one was dead and not accepting it the next moment. Isom said the overwhelming grief has worn on her.

“It’s more tender today,” Isom said of her own emotions, while on the verge of tears. “You’re in go mode, you help and then you finally feel it.”

Tim Craig and Jorge Ribas in Las Vegas and Amy Ellis Nutt in Washington contributed to this report.