Those families live with the memories every single day, and in Patricia Smith's case, they're fleeting.
"I don't have any independent memories of her," Smith told our sister station WABC in New York.
She was just a young child when she was robbed of the chance to grow up with her mother, NYPD Officer Moira Smith.
"It was hard to kind of hear everyone else's stories, but I didn't get that for myself," she said.
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Despite the memorials and mementos, the reality of remembering comes at the cost of resentment.
"It still makes me angry," Dr. Felix Torres said.
He is a clinical and forensic psychiatrist who has worked with survivors and families of loved ones who died in the attack, and he' seen the gamut of emotions. But for many, the most piercing and enduring emotion is the anger.
"Anger can manifest itself in many ways," he said. "It could be directed at other family members. It could be directed at the government, the terrorists, even themselves and their loved ones lost on that day."
For Smith and her dad, the anger is in the absence of justice
"Think we both kind of have the same notion that justice delayed is justice denied," she said. "And now we're 20 years later with no justice."
So while the old adage says that time heals all wounds, Dr. Torres will tell you it's not entirely accurate.
"Time itself is not a healer," he said.
The tributes, the services, and the annual ceremonies certainly hold space for healing for some, including Monica Iken, who lost her husband.
She visits the September 11th Memorial and Museum often, and her connection to it isn't just about honor.
"I couldn't bring him home, so this is his home for me," she said.
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But for others, it comes with a burden too heavy to bear.
"It may actually reopen those partially closed wounds," Dr. Torres said.
It is a search for healing and peace that for many will never come, no matter how many years have passed.