The instant Jim Asaiante heard the first explosion, he flashed back to Iraq.
"That was an IED," he said to no one in particular.
The former Army nurse fought the urge to rush toward the wounded. He knew there would be a second blast. Tending to soldiers blown up by roadside bombs had taught him that.
Asaiante paused and waited a few seconds. The ground shook again, a percussive explosion that sent more people scrambling, more smoke roiling down Boylston Street. While most ran for their lives, first responders like Asaiante swooped into action.
In the days to come, first responders would face down suspects in a massive manhunt that paralyzed a city. One officer would lose his life in a gunbattle, another would be critically wounded, while halfway across the country six firefighters and four emergency responders would be killed in a giant fertilizer plant explosion.
But in Boston, in that moment, helpers ranging from marathon volunteers to police officers to runners stopped in their tracks to help.
Asaiante had been a volunteer assigned to care for elite athletes in a tent just feet from the finish line. To him, the Boston Marathon was a rite of spring -- a way to celebrate everything great about the city.
He'd volunteered for 17 marathons starting in 1996 -- the 100th running of the iconic race. The only one he missed was in 2007, when he was working in combat hospitals in Iraq.
Asaiante had been paired with Stephen Segatore, a nurse from Tufts Medical Center. Up to that point, the two had treated runners mostly for dehydration. A few soldiers had blisters on their feet from running in full uniform, boots and all. One elite Kenyan was too cold; they gave him warm chicken broth.
Monday had been a perfect day. Clear skies with temperatures in the 40s and 50s, a relief after record highs nearly forced a cancellation of last year's race.
At 9 a.m., the nearly 25,000 runners observed a moment of silence for 26 seconds, in honor of the 26 people killed in the elementary school massacre in Newtown, Connecticut.
By 9:17 a.m., the race had begun with wheelchair participants, followed by the elite athletes. The rest of the runners were released in waves.
Fans who had gathered along Boylston witnessed a thrilling finish, with 23-year-old Lelisa Desisa of Ethiopia beating out two others in a sprint to the end. His winning time of 2 hours, 10 minutes, 22 seconds earned him the $150,000 winner's prize.
Over the next two hours, as the rest of the runners made that same glorious sprint -- the crème de la crème of marathon moments -- they were greeted by a cool sea breeze whipping down Boylston and jubilant fans rooting them on.
Near the finish line, Krystle Campbell, 29, had her eye on the race. Ever since she was a little girl, there was something about the resilience of people who huffed 26.2 miles that she found fascinating.
Ron Brassard was in the crowd, too. He'd come with his wife, daughter and others to watch a friend complete the race. They were soaking in the celebratory mood of Patriot's Day, a holiday he loved to share with family.
Less than a mile away, Horacio Hojman, the surgical director of the intensive care unit at Tufts Medical Center, was on call.
Then came the blasts. It was 2:50 p.m.
'We can't let this young girl die'
Asaiante and Segatore rushed from the medical tent to the finish line to tend to the wounded. The stench of burning flesh hung in the air. Blood pooled on the sidewalk.
People bleeding from lost limbs were already being carried toward the tent, so the two nurses stopped and headed back.
Runners were moved out of the area, and space was made for the wounded. Spanning the length of a football field, the tent was instantly turned into a massive triage center. It had 100 cots.
"No one was expecting they would have to treat trauma victims from an IED explosion," Asaiante said. "But you do what you have to do. You don't think. You just do."
Asaiante grabbed a roll of 6-inch elastic ACE bandage and used his shears to cut off a man's jeans. The man had suffered an arterial wound to his left leg and was in danger of losing the limb.
The nurse used the bandage as a makeshift tourniquet.