His family's search for meaning in the face of such grief is profiled in the September edition of The Atlantic.
Bobby's family sat down with 'Good Morning America' to discuss their lost loved one and how they continue to remember his legacy.
Bobby was 26 years old at the time. He had just started a job at Merrill Lynch that July.
"This beautiful life that he created - this plan to have this great life was no more. And that hit hard and still does to this day," Helen McIlvaine, Bobby's mother, said.
Bobby was one of the nearly 3,000 who were killed in the terrorist attacks when he headed to a conference in the North Tower.
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Bobby was born and raised in Oreland, Montgomery County. He graduated from Upper Dublin High School in 1993 and Princeton University in 1997.
He loved to write and kept diaries for most of his life, many filled with observations wise beyond his years.
"When he was 15 a good friend of mine died from cancer. He loved this woman. Many years later I read what he had written. 'I shall never forget those who died. Those who came before me. It's my job to remember them forever,'" Helen recalled. "It's kind of ironic because now he's gone and we need to remember him, as well."
A Brother, A Boyfriend & A Diary
Jeff McIlvaine, a high school teacher in Somerdale, New Jersey, said it's an amazing gift to be able to talk about his brother.
"My brother was a lot of things to a lot of people. He had an indescribable way about him that just made people want to be around him. Twenty years later, there are people who had small interactions with him that still remember him to this day," Jeff said.
Jennifer 'Jen' Middleton was Bobby's girlfriend back in September 2001. He was set to propose to her, as the last page of his final diary indicated, and start their life together before his was tragically cut short.
Jen kept that diary for all these years. In one passage, he wrote, "I miss Jen. She is my every day."
"It's been like a security blanket," Middleton said. "Knowing that his words and his writing are with me has brought me tremendous peace over the years."
The Atlantic staff writer Jennifer Senior said Bobby's story was very personal to her. Her brother was Bobby's roommate at Princeton for four straight years. Her brother then joined Bobby in New York as they navigated their new careers post-college together.
"We used to talk about writing all the time, Bobby and me. I was a writer. He wanted to be a writer," Jennifer Senior said.
Jennifer Senior said it was her mission to find Bobby's final journal. She said she knew it was the one diary that Bobby's family did not have.
The Atlantic writer said Bobby's father had given Jennifer the diary after his son died. His mother really wanted it back.
"There was some tension over who got ownership of that particular journal," Jennifer Senior said.
According to the article, Jen Middleton went into more details about the story behind the diary.
"In hindsight, I don't know what my problem was," Jen says. "I was probably in pain and also grasping for control and wanted something of his that no one else had. It seems kind of ridiculous now. It's just how I felt at the time-that it was mine and I wanted it to be mine and I didn't want anyone else to have it. It probably felt like it was all I had left."
She let Jennifer Senior borrow the journal, bring it to Bobby's family and said they could photocopy anything they wanted. She just wanted the original back, "no rush."
In the article, Jen said:
"I would have done it years ago. I think about them all the time."
But after all these years, Jennifer Middleton and Jeff McIlvaine were now in the same place.
"There were some problems with saying goodbye and the journal and that kept me from reaching out for many years, but when Jeff and I saw each other last night, it felt like no time had passed," Jen said. "It was just so wonderful. It warmed my heart. Water under the bridge. I'm so happy to be reconnected."
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The Next Generation
Jeff McIlvaine has four children. He makes sure to always bring up their Uncle Bobby.
"We talk about him constantly. It happens organically. We don't really sit down and say 'we're going to spend an hour talking about Uncle Bobby,'" Jeff said.
His oldest son is named Bobby in honor of his brother.
"Anytime I can share an anecdote or a story or something funny that he did, I make sure that I do that. It's a very safe place for me to share those stories without feeling like I'm burdening anybody; they enjoy hearing about them all the time," Jeff said.
Jen Middleton is now married with two kids, age 13 and 15. She said they talk about the attacks in their school and are old enough to understand what happened that day.
"I'm completely honest with them. We talk about it as we talk about anything else. My family is incredibly supportive," Jen said.
Read the full article from The Atlantic here.