(CNN) -- Kevin Conboy was backing out of his driveway for a midday burrito run with a friend when he heard the sirens.
He was driving down the street when he saw the flashing lights at Arapahoe High School.
That's when what Conboy calls "initial parent paranoia" set in.
"Are you ok" Conboy texted his high-schooler son, Ian, at 12:48 p.m. Friday, at first fearing not a shooting but something awry with his diabetic son's health.
"I'm fine," Ian texted back. But the school's on lockdown, he added. Something happened.
"Whoa," Conboy texted back.
What followed was a remarkable string of 70 more texts, displaying remarkable calm and the bond between a father and son amid a crisis that made national headlines.
Conboy, a Web developer and self-avowed "data sharer," said Monday he posted the texts to his website to revisit later, to write about more, to remind him on some distant day that "my relationship with my son is absolutely where I want it to be."
"We're in lockdown"
Friday's shooting began at 12:33 p.m., when, according to police accounts, 18-year-old student Karl Pierson walked into the Centennial, Colorado, school armed with a pump-action shotgun, a machete and a backpack containing three Molotov cocktails, a bandolier of ammunition across his chest.
According to police, Pierson shot 17-year-old Claire Esther Davis once in the head amid shots fired randomly into school hallways. He also ignited one of the firebombs in the school library before killing himself in the back corner of the school library.
Conboy didn't know any of that when he heard the sirens and saw the lights at the high school he once attended and where Ian now goes to school.
But he knew something was wrong. He steered the car not toward the Qdoba restaurant where he and a friend had planned to have lunch, but toward his son's school.
"Are you ok" he texted. "There are ambulances going to Arapahoe."
"I'm fine," the boy wrote back. "And yeah. We're in lockdown."
A constant dread
A school shooting has long been one of the characters in the cast of fears that dances through Conboy's mind when it comes to his three children.
He remembers being a young, young father -- barely out of high school -- cradling 1-year-old Ian and watching the horror of Columbine play out down the road from him in 1999.
The slaughter at Sandy Hook Elementary in Newtown, Connecticut, a year ago only intensified the dread.
He vowed back in 1999 he'd never have a gun in his home. And the plague of school shootings since then has only strengthened his belief that U.S. gun laws are, in his words, "severely underpowered."
But, on Friday, he wasn't thinking about gun laws. He had only one thing on his mind: his son.
"I am here and will be here"
Four minutes had passed since Conboy first texted his son.
"What's going on," he asked.
"Dunno," Ian wrote back. "We heard something like a gunshot and then an administrator came by to make sure the door was locked. Now we're all huddled in a corner."
"Please keep telling me you're ok," Conboy texted his son.
I'm good, the son replied, promising to text every five minutes.
"Ok I am here and will be here," Conboy texted. "Until I have you."
"Thanks dad. That Means a lot."
Dozens of texts followed. Can you get to the Burger King? No, Ian says. Go to Euclid Middle School. Actually, don't. Change of plans. Go to the Shepherd of the Hills Church instead.
I'm there, Dad says. You have to check me out, the son says. Standing in line, Dad answers.
"Okay thanks," Ian texts.
And then nothing for more than four hours.
The last entry is a photo -- Conboy and his son on the basement couch, alone, after Conboy had broken down in tears. After the rest of the family had gone from crying to celebrating. After everyone else had headed upstairs to decorate the Christmas tree.
"Oh my God," Conboy told CNN on Monday of the moment. "He's here, and what have we been through together."
A message on a hand
Another Arapahoe student, Matt Bowers, had his own message. It was written on his hand, in case he didn't get out alive.
"Family, I love you all so much," Bowers scrawled, the last two words underlined. Then he added, "I'm up here now," above a picture of a cross.
Bowers told CNN's The Lead that he was in a literature class when he and his classmates heard a loud bang from down the hall. He didn't think anything of it until they heard two more. His teacher's face "just became completely pale," he said, and everyone rushed to a corner of classroom away from doors and windows.
He's kept the ball-point pen he used to write that farewell ever since.
"I've never had such a frightening experience in my life before that," Bowers said. "It really changed how I looked at my life, and just my whole perspective on what life is."