A bullet had just pierced the wall of her classroom at Marjory Stoneman Douglas High School in Parkland, Florida. Gunshots -- a lot of them -- had exploded. Someone in her class was hurt. Someone else, somewhere, was yelling.

From under a teacher's desk, Hannah dialed 19-year-old Kaitlin at work.

Nothing.

So, Hannah did what teenagers do. She texted. And Kaitlin pinged back.

Hannah Carbocci, 17, left, and her sister, Kaitlin Carbocci, 19

Hannah Carbocci would not be among at least 17 people to die in Wednesday's massacre. But for several excruciating minutes, she and her sister thought she might be.

In those fragile moments, they traded 77 short text messages, which they later shared with CNN. The conversation offers an intimate glimpse into the panic, the fear and the helplessness that gripped a community that day.

On campus, students running from a madman thumbed last words of gratitude and love. Their relatives, out living ordinary days, dialed and redialed, texted and retexted, clinging to hope on their own smartphones.

Here is what Hannah, whose her sister calls her Boobles, and Kaitlin said in what they feared might be their very last exchange:

Hannah

kaitlin there is a shooter on campus

i am not joking

call 911 please

send them to douglas

Kaitlin

hannah what

are you serious rn

Hannah

kaitlin i am not joking they just shot through the walls someone in my class is injured

i am not joking

call mom and dad

i don't have service or i'd call

idk how this message is going through

Kaitlin

i'm calling

911

and then I'll call mom and dad

Hannah

please do thank you

Kaitlin

i love you so much

Hannah

im so scared i can't make any calls i love you

my phone service won't go through

Kaitlin

is police there

what's happening

hannah text me

Hannah

i don't know it's silent

i don't know if the police are here

Kaitlin

is it on lock down

Hannah

i think so

i hear yelling

Kaitlin

daddy is in response

he just called 911 they are on there way

Hannah

i am so scared kaitlin

tell them i love them so much

Kaitlin

i know Hannah

you're gonna be fine

Hannah

im so scared

Kaitlin

i know you are

daddy is on his way to the school

hannah keep texting me please

Hannah

tell them i love them so much

Kaitlin

Ii there anyone else shot

someone in your class got shot just now?

Hannah

just one person in my class room

they got shot like 8 minutes ago when the shooting started

Kaitlin

how many shots did you hear

Hannah

a lot

Kaitlin

Aad is the person ok

Hannah

the police are in the building

Kaitlin

ok how's the person who got shot

a lot of blood?

where did they get hit

Hannah

i don't know she isn't near me

i just know she got hit

Kaitlin

oh ok

Hannah

im so scared

im under my teachers desk i hear the police outside

Kaitlin

what class are you in

what building

Hannah

holocaust history first floor freshman building

Kaitlin

okay good so you'll be getting out first i hope

keep texting me

Hannah

i will

Kaitlin

Daddy said he's going to the school

and we can't get a hold of mom

Hannah

sorry i am texting you and gabriella too

it's okay as long as you have dad

Kaitlin

oh wow

Hannah

i am so scared

i hear the police in the hall

Kaitlin

good

they will help you

Hannah

i am so scared

Kaitlin

you'll be ok

you can't call me?

hannah

can you call me

Hannah

i have no service

Kaitlin

but you're texting me

try to please

Hannah

my service won't go through Android

Kaitlin

what

i have an iphone

you're talking to dad

you're safe

hannah

text me

did daddy get you

Hannah

i am with dad

Kaitlin

ok are you coming home now

Hannah

i think soim driving with daddy

Kaitlin

ok i love you so much

ash him if you're going home

Hannah

we are just driving right now

Kaitlin

ok