Man arrested in Highland Park massacre posted depiction of a mass shooting and imagery of parade site

• A man arrested after the July 4 mass shooting in Illinois posted violent videos online.
• He also posted imagery of the street where six people were shot dead and more were wounded.

The man arrested after the mass shooting at a July 4 parade in Highland Park, Illinois, had a history of posting violent imagery online — including a depiction of mass killing.

The police declared the 22-year-old a "person of interest" after a shooting in which a gunman positioned on a rooftop opened fire on a parade on the suburb's Park Avenue, leaving six dead and dozens injured.

After a search lasting several hours, police pulled over a car with the man inside about five miles from the scene of the shooting and arrested him. At the time of publication, he had not been charged with any offense.

The man arrested has a history of posting content with violent subject matters. He apparently went by a rap alias, according to an IMDB profile, and posted a series of videos with menacing themes.

The Associated Press said that he had a YouTube profile where he had posted videos depicting acts of violence.

One video Insider reviewed showed cartoon-style drawings of a mass shooting that seemed to take place in a school or similar setting. "It is my destiny," the video's narrator said. The YouTube page hosting the videos had been taken down by Tuesday morning, as had the man's Twitter account.

He had posted images of Central Avenue — the site of the shooting — in videos on his personal blog, which Insider also reviewed.

Police have not disclosed what they believe might have motivated the shooting. Nancy Rotering, the mayor of Highland Park, told told NBC News Tuesday morning that several of the arrested man's postings online "reflected a plan and a desire to commit carnage for a long time in advance."

"And it's one of those things where you step back and you say, 'What happened? How did somebody become this angry, this hateful?' To then take it out on innocent people who literally were just having a family day out," Rotering added.

Alex Newhouse, a researcher at the Middlebury Institute of International Studies at Monterey in California, said that man's online postings did not indicate any clear political affiliation, but rather immersion in violent and nihilistic online communities such as the 4Chan messaging board.

Some reports have drawn attention to posts the man made at Donald Trump rallies.

"The core part of this community is that its participants lose track of what is real and what is not. In a lot of cases, these people come from devoting themselves too deeply to alternative reality games and becoming convinced that they have dislocated from existence," he said.

The man's uncle told Fox 23 that he had seen "no signs of trouble" ahead of the shooting, and described his nephew as a "YouTube rapper."

The man arrested was brought up in the Central Avenue area, and his father is a local grocery-store owner and former mayoral candidate, according to local media reports.