Michael Bosse was standing 50 metres from the explosion that stained the Boston Marathon forever. He thought a power transformer had blown apart, but 12 seconds later, another explosion told him this was a terrorist attack.
By then he and half a dozen colleagues from Boston's Emergency Medical Services were running into the chaos, their training telling them to hold back, their instinct demanding they aid the injured and dying.
"The only way I could describe it: we were dealing with battlefield casualties," he says.
It was precisely six months ago.
Mr Bosse, the deputy superintendent of Boston EMS, the oldest ambulance service in the United States, is sharing his experience and the lessons learnt on that day with Australian emergency workers attending the Council of Ambulance Authorities' annual conference in Canberra. It's called Paramedicine of Tomorrow.
Mr Bosse is a veteran on the subject - he ran a field hospital at Ground Zero in New York City, arriving only hours after hijacked jets smashed into the World Trade Centre towers on September 11, 2001.
The response to the Boston Marathon bombings of April 15, 2013, has become the international standard for smooth operations in the face of terrorism.
The first critically injured patient was transported to hospital within nine minutes of the blasts; the last within an hour. In all, 118 people were transported by ambulance to one of the 13 hospitals and medical centres within reach.
Mr Bosse says he and his colleagues had an "unfair advantage". They were already on the scene when the bombs detonated, they had ample medical personnel and resources on site and they had ambulances - 37 basic life support units and eight advanced care trucks were used to ferry the injured. There were six Level 1 Trauma centres within five minutes of the blast zones.
The Boston Marathon swells the city's usual population of 650,000 to more than two million, and though, Mr Bosse says, "never in our wildest dreams did we imagine we would be attacked", the potential for a terrorist strike had long been been the basis of emergency service training.
What greeted Mr Bosse and his team when they reached the bomb scene remained intimidating. Amid the numerous people suffering shrapnel wounds and lacerations lay victims missing arms, legs, feet and hands, and some "completely dismembered".
As his team went to work, Mr Bosse's greatest fear was of another bomb.
"We are trained to anticipate that in a terrorist attack, there would be a delay after the initial explosion with a secondary device waiting to get the first responders," he said.
"A friend, a detective sergeant from the police, said another backpack had been detected across the street, directly opposite, and he said 'Mike, you've got to get out of here'.
"We stayed. We couldn't go out of there, we couldn't leave all these injured people."
Despite the shock and fear, bystanders removed their belts to fashion makeshift tourniquets and college students ran to a clothing shop, tossing out T-shirts to help stem the bleeding.
Mr Bosse's essential advice to those attending this week's conference in Canberra is straightforward: plan and prepare for the worst; train personnel well; practise drills constantly; and get to know the other main players such as firefighters and police so you're not meeting for the first time amid disaster.
When fire trucks blocked the path of ambulances, he knew precisely who to contact to sort it out fast: the fire chief, a drinking buddy.