"Then one of my people said, wow. They found his name, and it was probably ISIS or ISIS-related. Do you believe it?" Trump told supporters at a later rally in Kansas City, Missouri.
Trump also tweeted a video appearing to show Dimassimo, 22, dragging an American flag across a sidewalk, saying he had "ties to ISIS."
The footage in the video Trump tweeted appeared to be from a video Dimassimo had uploaded -- without the ISIS flag and playing different music -- to his YouTube page in April 2015. He wrote the video was taken from a protest at Wright State University supporting Eric Sheppard, a college student arrested last year for walking on the American flag.
“I thought it would ruffle some feathers, but I did not anticipate how tense the backlash would become,” Dimassimo told The Dayton Daily News last year. “If anything, all that has shown is that people in this area and people on the Internet care more about a symbolic piece of cloth, than they do a black person’s life … or, even beyond that, our Constitutional rights.”
When asked about the video Sunday on NBC's "Meet the Press," Trump defended the posting, though he was unable to definitively say anything about Dimassimo's alleged ISIS ties.
"All I know is what's on the internet," said Trump.
Nada Bakos, a former CIA targeting officer and analyst, said the video is "clearly dubbed over."
"Overlaying Arabic music does not make a terrorist video," she said.
Bakos said she worries about Trump’s judgment, saying what’s posted on the internet isn’t always accurate.
"It's disturbing to me that Donald Trump will take things at face value," she said. "If he's [president and] reading intelligence that comes to his desk ... that's imperfect information. How is he going to discern what's true and what's not?"
Bakos said ISIS wants to make Muslims in America feel unwanted and "Trump is playing perfectly into that strategy."
"I think they [ISIS] would be the last people that want to hurt him at this point,” she added.
ABC News has found nothing tying Dimassimo to ISIS. Former White House counter-terrorism advisor Richard Clarke said it's unlikely Dimassimo would have been released from jail if he had ties to the terror group.
"If the federal government thought he was a supporter of ISIS, it is unlikely he would be allowed to walk," said Clarke, an ABC News consultant.
The Secret Service, FBI, and Department of Homeland Security have not yet responded to requests for comment on Trump's allegation.
Dimassimo was released on bail Saturday evening. It was not clear if he entered a plea or had an attorney.
ABC News was unable to find a phone number listed to Dimassimo. Calls to his parents weren't returned.