It's the split-second choices -- left or right -- that haunt the medical first responders to the 9/11 attacks on the World Trade Centre.
Fifteen years later, those decisions have not left them because that decision was the difference between life and death.
The medics have found an unlikely person to share their fears, hopes and illnesses with -- an Australian paramedic teacher and researcher.
Edith Cowan University doctor Erin Smith told The Huffington Post Australia she started a 13-year study documenting the physical and psychological effects of 9/11 medic first responders after a meeting with a widow.
"My research began in a Brooklyn cafe in 2003 when I was meeting with an incredible woman called Marian Fontana, whose firefighter husband Dave died when the first tower fell," Smith told HuffPost Australia.
"She was reflecting on whether the experience would have changed Dave if he had survived. Whether he'd be haunted by what he saw."
Fontana told Smith there wasn't much to be done for her family, but the survivors needed help.
"She said 'help the survivors that are still here struggling with that daily, ongoing guilt'."
Smith went on to interview 54 medics over the 15 years since the terrorist attack.
Smith said medics wanted their stories told, but those first interviews weren't easy for them.
"In the U.S. there was a lot of focus on the firefighters, I think because there were more of them, so among the medics, they hadn't really had a chance to tell their stories," Smith said.
"Many medics that responded on 9/11 feel as if their plight has been forgotten. I have had a few medics ask me why it took someone from the other side of the world to come and listen to our stories.
"Back then, they were still overwhelmingly impacted by the experience. What they saw. What it sounded like. What they could smell.
It's their job and their duty and they'd put themselves in harm's way again in a heartbeat.Erin Smith
"They got very emotional and kept apologising or losing their train of thought, but what they kept coming back to the sights and experiences.
"They couldn't move past it."
At that early time, she asked them if it happened again, would they do the same thing again.
"Overwhelmingly, they said yes. It's their job and their duty and they'd put themselves in harm's way again in a heartbeat."
Yet their lives have not been easy. Of the group, 13 have been diagnosed with exposure-related cancers, one has died and two are terminally ill.
Smith said a lot of people didn't realise that it wasn't just one day breathing in toxic dust, it was a nine-month process of rescue and recovery at Ground Zero.
"In the beginning, no one had the right protective breathing gear," Smith said.
"They've all got respiratory conditions and the cancer rate among first responders is triple that of the population of Manhattan and there were also asbestos fibres in the dust so there will still be years before we see cases of mesothelioma."
As such, the death toll for first responder medics related to September 11 continues to increase every year.
John Feal was one of the thousands of first responders involved in the long search and rescue and debris removal operation at Ground Zero.
He said it was vital that the names of first responders who died as a result of their service all be remembered at the 9/11 Responders Remembered Park in Nesconset, New York.
"The park speaks for itself," Feal said.
"It is a reminder of how great our country can be. And the park will tell our story so history is never distorted."
As for Smith's research, she said lessons could be learnt for Australian emergency services -- both with the ways emergency services communicated across police, fire and ambulance during a mass-casualty event but also with supporting then after the fact.
"I'm also going to research further how spouses and loved ones have been affected because anecdotally, a lot of the medics I spoke to said they were worried for their children or spouse because they'd been unhappy for so many years.
"The next step is researching how spouses can be supported as well."