"Blood everywhere. Blood all over the floor. The stench of blood and bodies whisking by in either direction on stretchers," he recalled. "Just a mind-boggling sight that you're not expecting and not prepared for."
Sunrise was one of the closest hospitals to Las Vegas Village, where 22,000 people had gathered for a country music festival, only to scatter in panic after a gunman perched 32 floors above them began raining automatic gunfire onto the crowd.
Within minutes, nearly 500 people were wounded. Soon after that, an impromptu fleet of ambulances, private vehicles, taxis, Ubers and Lyfts began scooping them up from the festival site on the Strip and rushing them to nearby hospitals, where besieged emergency rooms became the front lines in a battle to save lives.
About 200 of the injured went to Sunrise, most of them with gunshot or shrapnel wounds.
"It was organized chaos," said Seiff, 55, who has worked at Sunrise since 2001
. As chief of neurosurgery, he had seen all the injuries in the past and smelled the "pungent, metallic" odor of blood.
But nothing like this. Not on this scale.
"Everything I saw on Sunday night I've seen before, just not at one time," he said.
Injured people filled the waiting room and hallways. Nurses and doctors frantically scurried to triage patients so the most desperate could be treated first.
Nineteen patients -- all head and spine injuries -- came to Seiff. He and Drs. John Anson, Keith Blum and Gene Khavkin handled 10 head-shot victims and nine spinal injuries. Fifteen of them made it, though one patient will be a quadriplegic, two others paraplegic.
"That's a devastating thing to have to tell somebody," Seiff said.
A bullet had passed through both of one patient's shoulders. She couldn't move her legs. The neurosurgeons quickly had to make room for her spine to swell.
A woman in her 20s had been shot through the eye. The base of her skull was smashed. Another young woman had a bullet wound to her jaw.
Briefly, Seiff pondered the women's disfigurement but pushed the thought away.
"That wasn't the time to think about those kinds of things," he said. "Emotion wells up, and you realize you could start crying, but you force it back down. It's not the time."
Code red = patient near death
Across the city that night, the phones of hundreds of Las Vegas medical professionals began buzzing and dinging. Doctors, nurses and other hospital staff -- some of whom had just left work -- were alerted to a "mass casualty event."
Most had gotten similar texts before -- after bad car wrecks.
Dr. Scott Scherr, 43, medical director for Sunrise's emergency department, had just climbed into bed to go to sleep when he received some 20 texts and five phone calls in just a few minutes.
That was unprecedented, much like what he was about to see.
He returned to the hospital to find patients pouring out of ambulances, taxis and private vehicles. Friends dragged listless bodies into the emergency room. Hospital personnel waved badges from their car windows as security guards tried to direct them through the mayhem.
Within about 90 minutes, the 45-bed emergency department was at more than quadruple its capacity. The chaos overflowed into the pediatric and post-anesthesia units, as doctors and nurses color-coded patients in the hallways and waiting room.
Red meant dying. Those patients were sent to trauma bays or operating rooms. Yellow meant life-threatening injuries but the patient had an hour or so to live. They could wait. Green was for walking wounded.
One Sunrise doctor who served in Iraq and Afghanistan told Scherr it was like "something you would see in a war zone."
Nurse practitioners and physician assistants kept reevaluating the patients constantly to make sure yellows didn't become reds. Meanwhile orthopedic doctors were treating gunshot wounds and pediatric doctors were operating on adults.
On a normal evening at Sunrise, there are eight people staffing the emergency room. On Sunday night, Scherr said, that swelled to 200.
"We deal with blood a lot, but we don't deal with this magnitude of blood loss all at once," said Scherr. In a busy week, he said, his emergency room might typically see 6-7 shooting victims.
When he left the hospital after 20 hours treating people, Sunrise had administered 300 units of blood, he said -- almost 20 gallons.
'There's not a time for me to feel'
A few miles away, the University Medical Center of Southern Nevada's Level 1 trauma center -- the only one in the state -- had surged from a norm of 12 to 20 staffers to about 200, spokeswoman Danita Cohen said.
More than 100 patients came through its doors. Dr. John Fildes, the hospital's medical director, told CNN it was the largest wave of mass casualties they had ever seen.
"The alarming thing was watching the front door open, and it never stopped. And gurneys just kept coming in," he said. "Trucks would pull up, people would be brought in by friends."
Like so many doctors, Dr. Paul Chestovich, 39, a trauma surgeon whose specialty is major bleeding, was at home when he got the alert.
His anxiety soared as he raced to the hospital, and his adrenaline spiked when he saw the bodies and blood.
"You take that nervousness and you translate it into energy of action," he said.
Complicating matters was that the trauma resuscitation unit already had nine patients, including a burn victim and a pedestrian who'd been hit by a car, said clinical nurse supervisor Brad Skilling.
The unit was two-thirds full, and yet 39 critically injured patients were about to be wheeled in.
Because the shooter had used high-velocity rounds, Chestovich's skill set was in demand. These types of bullets, he said, carry an enormous amount of energy, which is "transmitted to whatever stops the projectile," he said.
Chestovich focused on stopping bleeding, along with the "ABCDE" of trauma care: airway, breathing, circulation, disability and exposure.
"I had to decide what's going to kill people first and fix that," the surgeon said.
Skilling and his counterpart, Toni Mullan, were also called in to help with the response. Mullan had just finished a 12-hour shift and was in the bath when she got the call.
She had to stave off her emotions upon seeing the carnage.
"To say 'chaos' isn't telling you how the scene was," she said. "There's not a time for me to feel. ... You need to figure out what was going to work."
Triage began outside, between the police barricades and the ER's front door, as a nurse and a doctor directed patients -- limping, in wheelchairs, on gurneys -- to treatment. Skilling and Mullan orchestrated blood, blankets, medicine, CT scans, nurses, lab techs, respiratory therapists -- you name it -- to make sure patients received what they needed.
"My job is to figure out how to calm the storm and how to direct it," said Skilling, who has worked in trauma since 1993.
"I did look around. I took a deep breath. I guess I've been doing it so long I don't get real fazed by it. Back in the day, I did."
Five victims in one pickup truck
If a dozen Las Vegas area hospitals -- Sunrise and UMC chief among them -- hadn't been ready to treat the hundreds of injured, it's possible that far more than 58 people could have died.
But those doctors and physicians aren't the only heroes. Civilians played a crucial role in ferrying the injured to help.
According to witnesses, while some frightened concertgoers refused to stop as they escaped the scene in vehicles, others lined up along the shoulder to take victims to hospitals.
Lindsay Padgett, 29, and fiancée Mark Jay, 37, were among the lucky people to make it out unharmed.
They were exiting a parking lot, gaping as shell-shocked pedestrians meandered through the crowd and people ran by with bodies in wheelbarrows and on police barricades repurposed as gurneys.
A man ran up to their passenger-side window and said, "Right now, we need your truck. We just need to get people over to the hospital, OK?"
They piled five victims -- one of them a young man with a faint heartbeat, his limp body slumped over another man's shoulder -- into Padgett's white pickup, along with seven of their friends.
"That is just something that you do," Padgett said. "You take people to the hospital. You put them in the car."
They were able to get three of the victims to paramedics. They took a fourth, a Canadian woman who had been shot in the leg, to Valley Hospital Medical Center. The young man with the faint heartbeat didn't make it.
In a Facebook Live video she shot outside the hospital, Padgett said, "We've got to go back and get more people. Some guy just died in the back of my truck." She exhaled and wiped tears from her eyes.
'There was a lot of selflessness that night'
Also in the crowd that night were Las Vegas paramedic and firefighter Ben Kole, Clark County fireman Jesse Gomez and Henderson fireman Anthony Robone.
They're among the many first responders who spent their days off at the music festival. Twelve firefighters were shot at the concert, two while performing CPR, said Las Vegas Firefighters Local 1285 President Eric Littmann.
When the bullets began coming down, Robone, 25, covered his girlfriend and told her everything was fine, even though he was not sure that was true. Then he heard his brother, Nick, say, "I got hit," and saw him spit up blood.
Robone and a friend got his brother into a Sunrise-bound ambulance and then "started trying to treat patients as best we could in the street."
Robone saw people taking off belts to make tourniquets, strangers applying pressure to victims' wounds and a person who was shot in the arm get out of an ambulance to make room for someone more critically injured.
"There was a lot of selflessness that night," he said.
Gomez, 45, got his family to safety, then told his wife, Debbie, to go home without him.
"She begged me not to go. It was probably one of the hardest decisions I've ever made, but she knew I had to go," he said, adding that his wife ended up taking one of the victims to Spring Valley Hospital.
Gomez also struggled with telling people staying behind with deceased friends and kin that they had to evacuate and leave their loved ones behind.
For Kole, 50, it was especially agonizing to have to tell those near death that he couldn't do anything for them.
"You have to tell them, 'I'm sorry. I have to help somebody else,'" he said. "You don't want to give them false hope, and you want to give those that are serviceable a chance."
He and his daughter, Rachel, an EMT who had been assigned to the concert, worked in tandem, triaging the injured. Kole estimates that between them they had to deliver that devastating news to a half dozen victims.
But they also were able to treat about 50 people, he said.
Rachel Kole is 20 and has been an EMT only since May. Amid all the agony, she also saw plenty of altruism.
One victim who'd been shot in the arm told her, "You can move on. There are other people who need you more."
'I'm emotionally blunted'
Now, the healers must heal. If their long stares, clasped fingers, quavering voices and exhausted sighs are indications, it could be a long process.
Scherr, the emergency room director, said he broke down after leaving the hospital. He watched news footage of concertgoers fleeing the gunshots and "I could just imagine the fear of those people," he said.
He wept again later, but for a different reason: He realized, "Our team did a fantastic job and we saved a lot of lives."
Seiff, the neurosurgeon, hadn't cried as of Thursday. But he came close when, at his 17-year-old daughter's behest, he wrote his first Facebook post in more than a year to let everyone know he was all right.
"Friends'll say I'm emotionally blunted," he told CNN. "I don't know. It could be something innate that allows me to do what I do."
In the last few days, Seiff has gotten some welcome news. He visited at least six of his patients Wednesday and they were stable. The woman who couldn't move her legs Sunday night moved them Tuesday. The woman shot through the eye may go home this weekend.
A brief smile illuminated his weary face as he walked to his car following an interview at a local bagel shop.
Padgett and Jay, who evacuated five victims, are still trying to process what they witnessed. Each day, it seems, they remember something new about the attack.
"It's like a nightmare, like it's not real," Padgett said as Jay drove her truck, still streaked with blood, to a local car wash that was offering free detailing to people who had helped victims.
There, the sound of water from the power washers hitting the hood of the truck made Padgett visibly shudder.
"It just triggers something and makes you think about it," she said. "I can't explain it."
Ben Kole and his daughter, meanwhile, have already been to a therapy session together. Kole wasn't sure counseling was for him, but he wanted to make sure Rachel healed, he said.
At their first session, a counselor told him something that resonated: If he and his daughter chose to live in fear, then the shooter had claimed two more victims.
Before they began treating victims on Sunday, the pair frantically searched for each other amid the bedlam of the festival grounds. After it was all over, they came together and wept.
And like many people who pitched in that night, they are still grappling with the horrors they witnessed.
"You wake up the next morning and go, 'Wow, I'm still here," said Rachel Kole, a pre-med student at the University of Nevada-Las Vegas. "And you sit on the couch, and you almost don't know what to do -- how to continue."
But Rachel told CNN her dad's courage in the face of danger is inspiring her. "To see him in action encouraged me to do my best and one day be able to perform as well as he does."