Kimberly Rex: My father was killed on 9/11. I still struggle to understand how he just disappeared.

On Sept. 10, 2001, I ate dinner beside my father in our Staten Island, New York, home. I was 19 and sat at his left, as usual. I watched him shake spoonfuls of grated cheese onto his soup. He was right next to me: flesh and bones, salt-and-pepper hair and a sharp nose.

The next day, the plane hit. Fire raged and smoke billowed. Then the floor where he stood, the walls, the ceilings and the windows crumbled away into dust.

And the people inside disappeared.

At home, we thought at first that they were only missing. We made posters with names, faces and phone numbers. Such fliers covered windows and storefronts, wrapped around bus stops and bodegas. They flapped in the wind of empty streets, begging anyone to find those lost and bring them home.

We sat around tables and made phone calls. We lit candles and prayed in circles on front lawns. Cars stopped, and strangers climbed out to join our vigil. If we prayed hard enough, it seemed, maybe he’d feel it and come back to us.

One by one, we knew he wouldn’t. My mother, my three sisters, his parents and friends each knew in time that he was dead. But knowing that didn’t change the feeling that he was missing.

Later, we were given back pieces of him. We knew we were lucky to be able to bury a part of him, but he was still lost.

My father’s absence hung over our family. On my birthday two months later, there was a painful blank space underneath where my mother had written “Love you, Mommy” on my card. Even his name was gone. The following June, my grandfather fell ill. He cried on his deathbed for his son who died in an inferno. At his funeral, my grandmother dropped a thorny rose onto his grave and said, “Find my Vincent and tell him I love him.”

On the first anniversary of 9/11, we loved ones piled into buses and rode to Ground Zero. The wind that day was brutal – 35 mph at one point – pushing our bodies and pulling our clothes and hair as we walked down the ramp into the pit.

On that hallowed ground, a weight settled inside my chest. In that space, in the whipping air, something else was there: the missing.

We felt them in our steps, our bones, the wind.

For the few minutes that my mother, sisters and I bunched ourselves into a hug and sobbed, it felt as though we had found my father. Finally. But once we climbed back up to the city sidewalk, he was gone again.

In the 20 years since, I’ve never found him again. He’s on my mind, of course, during the special moments but also the mundane. I tell stories about him, like the time I woke him up while attempting to hang a bulletin board in my teenage bedroom. I was bending every nail I tried to hammer. He walked in, hair askew, took the hammer from my hand, drove the nail into the wall with two whacks and left without a word. Sometimes, memories like this make me smile. Other times, I want to cry for all that he has missed – for all that I have missed without him.

Sometimes I hear myself sound like other people whose dads have died. But then I hurt again, because my father didn’t just die. He vanished. And it’s not the same when death is unseen and uncertain; when loved ones are robbed of the chance to prepare and accept; when his sneakers still sit in the front closet where he’d slipped them off and the smell of his aftershave still lingers in the bathroom.

Twenty long years have passed since that painful day. So much of my life has happened. I graduated college, taught high school English for eight years, met and married my husband. I’ve been gravely sick and recovered. I adopted two babies and watched them grow into lovely little girls. So many things to feel happy about. But I’ve never stopped struggling to understand how my father’s strong, stocky body just went away, disintegrated. The need to find him has never left – nor the sadness of knowing I never will.

Kimberly Rex is a writer in New York. Her father, Vincent Litto, a vice president at Cantor Fitzgerald, was killed on Sept. 11, 2001.