Highland Park shooting suspect Robert Crimo has bizarre obsession with the number 47

Suspected gunman left extensive digital footprint in which he frequently referenced the number 47

Robert Crimo has a tattoo of the number 47 on the side of his face (Robert Crimo via REUTERS)

The 21-year-old suspected of opening fire on the Highland Park 4th of July parade appears to have an obsession with the number 47.

Police believe Robert “Bobby” Crimo III dressed in a wig and women’s clothes before using a high-powered rifle to fire more than 70 rounds into a crowd in the Chicago suburb on Monday morning, killing six and wounding dozens.

He is expected to be charged Tuesday.

Mr Crimo left an extensive digital footprint in which the aspiring rapper, who performed under the alter-ego Awake, frequently referenced the number 47.

He also had the number tattooed on the side of his face, and it was also painted on the door of a car parked outside his family home.

Social media users pointed to a photograph of Mr Crimo wearing a dark suit with a red tie, which appeared to closely resemble the character Agent 47 from the popular first-person shooter video game Hitman.

“Has anyone figured out that the 47 tattoo is a part of his violent fantasy world?” Andrew Chafin asked on Twitter.

The date of the Highland Park shooting on 4/7 was also pointed to as another potentially significant reference among commenters.

Mr Crimo had also turned up to a rally in support of Donald Trump, who boasted he would become the “45th and 47th” president while playing a round of gold in January.

Mr Crimo’s lyrics and social media posts also feature references to mass shootings, and appeared to glamorise mass violence.

A Twitter account associated with Robert Crimo posted the number 47 dozens of times (Twitter)

NBC reported that his last post to YouTube prior to the killing was of the aftermath of a school shooting.

He was also a regular poster to Discord, where he shared videos of beheadings, suicide and murder.

Police have said they are yet to identify a motive in the shooting.

In an interview on The Today Show on Tuesday, Highland Park Mayor Nancy Rotering said Mr Crimo’s many, disturbing posts online were not something she wanted to focus on.

“We know that several postings really reflected a plan and a desire to commit carnage for a long time in advance,” she said.

“But I’m hesitant, frankly...to talk about that. I don’t want other people to think this is something that needs to be glorified. This is something that needs to be diminished.”

Ms Rotering said she would prefer to focus on enacting gun control measures.

The weapon used in the shooting had been legally purchased, authorities said.