He was in New York City on business for one day only and seated on the 105th floor of the World Trade Center's South Tower when a hijacked plane hit the first tower.
"It feels like it was yesterday in some ways, and in other ways it feels like it's been forever," he said.
The 61-year-old has been in the insurance industry for 42 years and said it wasn't unusual to be called to the World Trade Center in his line of work.
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When a hijacked plane hit the first tower, he said he didn't hear or see anything from the 105th floor.
"All we saw was a flicker of lights," Dittmar said.
The room he was in was surrounded by four walls, no windows.
At that point, he, along with the 54 others in the room, were told to evacuate because of an explosion.
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He made his way down the stairwell, but once on the 78th floor he said he had a choice to take the elevator or continue down the stairs.
Knowing a bit about fire safety, he took the stairs.
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"I was a very, very fortunate man," he said.
It wasn't until he reached the lobby of the tower that he saw the scope of what happened.
On the ground he saw twisted steel, crumbled concrete and "big red markings" on the ground - but it's not those images that keep him up night, Ditmarr said.
It's the sounds of screams.
"The sounds of hundreds of thousands of people all screaming the same scream on the streets of New York as the south tower came down, that's the sound I hear first thing in the morning and last thing at night," he said.
It wasn't until seven hours later he was able to reach his wife, back home in Illinois, by phone.
"(It was) one of the greatest phone calls of my life," the father of four said.
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Nineteen years later, Ditmarr says the events of Sept. 11, 2001 need to be remembered.
"It's my obligation to tell the story," he said.
Ditmarr now lives in Delaware and still works in the insurance business, though what he saw that day still lives on in his memory, especially on the anniversary of the tragedy.
"The memories and things we saw and felt that day, they come back to us, particularly this time every year," he said.
He still goes back to New York City for business and commemoration of the attacks.
"I cry every time," he said.
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The tears are for the people who lost their lives who should've survived, he said.
As a final thought, Ditmarr encourages Americans to live life to the fullest, even during times of hardship or tragedy.
"You can't take any day for granted," he said.
Ditmarr has since started the Always Remember Initiative in honor of the events of Sept. 11, 2001. To make a donation, click here.
VIDEOS: News coverage on Sept. 11, 2001 from WABC-TV, our sister station in NYC