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The father of a man accused of mass murder at a Fourth of July parade has pleaded guilty to reckless conduct for helping his son obtain the gun allegedly used in the attack.

Robert Crimo Jr was sentenced to 60 days in jail, two years of probation and 100 hours of community service as part of a plea deal.

Crimo Jr sponsored a firearms ownership card for his son, Robert Crimo III, despite the son's history of mental illness and threats of violence.

Police were called to the Crimo household at least twice in 2019, once after a suicide attempt by Robert Crimo III, and another time when he threatened to kill family members. Officers confiscated knives, according to police reports, but did not charge anyone with a crime in connection with the incidents.

The elder Crimo sponsored his son's firearms ownership card several months after the police calls. Robert Crimo III was 19 at the time. Illinois residents under age 21 need parental permission to obtain a card.

Mr Crimo III went on to use the card to buy several guns over the next several years, including one that police say he used to kill seven people and injure dozens at a holiday parade on 4 July 2022 in Highland Park, a suburb north of Chicago. A date for his trial will be set later this year.

Prosecutions of parents of alleged mass shooters are rare.

In one notable case, the parents of a teenager who shot dead four classmates in Michigan will go on trial on charges of involuntary manslaughter next year. And an Illinois man who gave his son a rifle that he later used in a mass shooting in Tennessee in March.

Before entering his guilty plea, part of a last-minute deal with prosecutors, the elder Crimo had been scheduled to go on trial starting Monday on seven reckless conduct charges, each carrying a maximum sentence of three years in prison.

His attorney, George Gomez, told a news conference that the plea deal was in the best interests of the community as well as the younger Crimo's trial defence.

"The last thing Mr Crimo wants is the Highland Park community to relive these tragic events and make a public spectacle," Mr Gomez told reporters.

If Crimo Jr stood trial, the lawyer said, evidence introduced could have jeopardised his son's legal case.

"As a father, Mr Crimo wanted to ensure that his son received a fair trial," Mr Gomez said.