Robert Crimo was taken into custody on Monday night following an eight-hour manhunt after a gunman killed six and wounded dozens more during Independence Day celebrations in the Illinois city.
Mr Crimo, who has not been formally charged over the attack, appears to be a supporter of the former president – who is currently himself the focus of an investigation in Congress over the January 6 Capitol riot.
The 22-year-old, who posted disturbing videos online ahead of Monday’s massacre, was pictured in the Waldo outfit at a Trump rally in Northbrook, Illinois, in September 2020.
He posted a selfie of himself at the event with a Trump 2020 flag seen in the background.
Mr Crimo was also pictured in aChicago Tribune story about the rally, where the protest was described as “tense” with supporters and counter-protesters facing off against each other.
Another photo circulating online shows Mr Crimo wrapped in a blue Mr Trump flag.
It has also emerged that Mr Crimo’s father, Bob Crimo, only follows one account on Twitter – an archived account of Mr Trump’s tweets.
Mayor Nancy Rotering, who took on and beat Mr Crimo’s father Bob Crimo for the role, said on Tuesday that she expects the 22-year-old to be charged over the July 4 attack later today.
Prior to the massacre, Mr Crimo had posted several disturbing videos glorifying violence on his YouTube account.
He is also an amateur rapper who goes by the name Awake the Rapper and has more than 16,000 listeners a month on his Spotify page.
His uncle spoke out to CNN saying that he had seen “no signs” to suggest that his reclusive nephew would go on to carry out a mass shooting.
The gunman is believed to have scaled a ladder on the side of the building and taken up a sniper position on the roof before opening fire at around 10.14am on Monday morning – just 14 minutes into the July 4 parade that had drawn local residents and families.
The weapon was recovered from the roof of a business overlooking the parade route.
Officials are keeping quiet about the exact type of gun that was used in the massacre, only saying that it was a “high-powered” long rifle that was used to “spray” bullets into the crowd.
On Tuesday morning, Mayor Rottering confirmed that the firearm used in the attack was legally obtained.
Five adults died on the scene of the mass shooting while a sixth victim died after being rushed to hospital.
The first details have begun to emerge about the victims with the family of 76-year-old grandfather Nicholas Toledo confirming his death on Monday afternoon.
The elderly man, of dual US and Mexico nationality, used a wheelchair and was sitting in it when he was struck by bullets.
Mr Toledo’s granddaughter told the New York Times they “were all in shock” and revealed that her grandfather hadn’t wanted to go to the parade but did so to join his family.
A second victim was also identified on Monday night as a devoted member of a local synagogue.
Jacki Sundheim was remembered by the North Shore Congregation Israel as a “lifelong” congregant and “cherished” staffer whose “work, kindness and warmth touched us all”.
As well as the six victims killed, 38 people were taken to local hospitals for treatment.
By Tuesday morning, eight patients were still receiving treatment with their conditions currently unknown.
President Joe Biden condemned the shooting and said that it showed there was “much more work to do” to tackle America’s gun violence.