(CNN) - The first tweet came at 2:59 p.m., sober and startling.
"I am in a school shooting right now..."
As the world soon learned, it was not a sick joke. A 14-year-old student at a South Florida high school was hiding under a desk in a classroom, sharing his fears on Twitter as a gunman stalked the halls.
And for the next 86 agonizing minutes freshman Aidan Minoff gave the world a glimpse into Wednesday's horrific shooting at Marjory Stoneman Douglas High School, which killed 17 students and faculty.
Here's a timeline of his terror:
Aidan told the Miami Herald that after the school's fire alarm went off and he heard a series of popping sounds, he and other students were herded into a classroom. A teacher turned the lights off, and they hunkered down.
"We thought maybe it was firecrackers," he said, "until we heard screaming and ... they were barricading the door. We knew it wasn't a joke anymore."
After telling his family that he was OK, Aidan began tweeting.
Two minutes after his first tweet, he tweeted again.
"My school is being shot up and I am locked inside. I'm f***ing scared right now."
He included two photos, taken inside a classroom. Shot from a low angle, they show students sitting on the floor between desks, checking their phones. One appears to be texting.
At this point, according to police, the gunman was still at large.
A swelling chorus of people began tweeting at Aidan, begging him to stay safe.
Nine minutes later came his next tweet, with a similar photo.
"Still locked in. I checked the local news and there is 20 victims. Long live Majory Stoneman Douglas High."
By now, Aidan's Twitter feed was overrun with messages from well-wishers and news reporters. He remained silent for 35 excruciating minutes.
Finally, he tweeted again.
"Hello, Twitter. I am closing my DM's but I appreciate everyone contacting me. I am still locked in the school, but remember I'm only a freshman. Please don't just send your love to me, but pray for the victims' families too. Love you all."
Authorities arrested suspected shooter Nikolas Cruz in a nearby neighborhood.
Fifteen minutes later, Aidan tweeted a virtual sigh of relief.
"We have been liberated," he wrote, along with a photo snapped outside the school. "God bless, America."
Half an hour later, the teen posted another photo showing people milling around outside the school, along with emergency vehicles and yellow crime-scene tape.
With the photo, he included a message:
"Love each other. You may never know when it may be the last day you meet someone."
Later, Aidan talked to CNN's Don Lemon about why he live-tweeted the ordeal.
"I was nervous and I just wanted it (the shooting) to be known, it was out there. And I started getting notifications on my phone that people were engaging with the tweet and people really found it useful information," he said.
"You know, there was no major news source at the time. There was barely any police there at the time. So I feel like a lot of people found it useful and I'm glad that I could do that."
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