Every coo and every cry was a reminder that life goes on.
As the site of this tragic disaster rebuilds and takes shape, "World News" checked in with some of the children of 9/11 to hear the stories they carry in their hearts, nine years after losing their fathers.
There was Terilyn Patrick and her newborn son, Jack, born on Halloween in 2001. Terilyn's husband, Jim, was a bond broker who worked on the 105th floor of Tower One. Two days before his death, Jim and Terilyn had celebrated their first wedding anniversary.
In 2001, she said: "It feels more real now that the baby's here. It's put reality right in front of your face. Plus every time I look at him, I think of Jim."
Two years later, Jack, by then old enough to talk, was seen in a home video speaking about his father.
"Where does daddy live?" Terilyn asks on the tape.
"Heaven," Jack responds.
"Who does he live with now?" his mother asks.
"God. And in my heart," Jack says.
Today, Jack has his father's eyes and his love for the outdoors. He could not find the words to express himself when asked about his father. He cried as his mother wiped away his tears.
"Moving on means we live our life to the fullest and live in a way that would make Jim proud," Terilyn said. "We remember him and make him proud. We are no longer victims."
ABC News was present in 2001 as Vycki Higley gave birth to Robyn, named after father Rob, with family and friends in the delivery room. Rob died while trying to help others escape the burning towers. Vycki said the last nine years have been a rollercoaster of emotions.
"He was a wonderful, fun-loving guy. He loved life. He loved his children. He loved his friends and he had a horrendous sense of humor," Vycki said.
Today, Vycki and Robyn keep Rob in reach with reminders all over the house. There are the Twin Towers in the kitchen and living room and a scrapbook full of memories of Rob. "I know how much he was looking forward to having another child," Vycki said. "It gives me something to hold on to."
"Not too many of Robyn's friends know. Friends and their families are still catching on to what we've been through, which is kind of strange in a way," Vycki said.
"Before I go to bed, when my mom leaves, I think about him and it makes me feel happy and safe and it really helps me feel better," Robyn said. "When I'm upset, I go to the basketball court and think about him and I feel like he's there with me."
Holly O'Neill learned quickly in 2006, when daughter Sean -- named after her father -- began to ask questions, that old answers weren't enough for 4-year-olds.
"She said, 'What happened?' You know, 'What happened?' And I sort of gave her a very simple chronology," Holly said. "'When we came home, you were in my belly and we were both really excited and there were some bad men and they set a fire to the building that your daddy worked in and, and I said, and you know, he went to heaven that day along with a lot of other daddies and mommies.' And she said, 'How come I'm the only one, I'm the only one without a daddy?" And then I just tell her, actually, that's not true."
Sean has kept a photo of her father on her nightstand since she was two.
"I just love having it near my bed because it's an easier way to keep him near me," she said. "Usually I sleep with two pillows by my side pretending it was him and my mom. It's just so special to me."
"She's mourning the loss of someone she never knew," her mother said. "It's so hard to get a child's head around that."
Sean said no one feels the way she does. "They just say, 'Well, my grandpa died.' And I say, 'It's not the same thing.'"