The father of the suspect in the deadly shooting at a Fourth of July parade in suburban Chicago last year wants the judge in the case to drop criminal charges accusing him of helping his son obtain a gun license before the massacre. 

Robert Crimo Jr.'s lawyer, George Gomez, told the court during a hearing in Waukegan Monday that his client was charged under a law that is vague and unconstitutional. He said prosecutors charged Crimo Jr. for "innocent conduct" — signing "a wholly truthful" affidavit helping his son, Robert Crimo III, apply for and obtain a gun license.

Legal experts have said charges against parents following mass shootings are rare partially because the cases are difficult to prove. 

Crimo Jr. has pleaded not guilty to seven counts of reckless conduct, one for each person killed. Each count carries a maximum three-year prison term. Prosecutors allege the father aided the son in getting a gun license even though he knew the then-19-year-old had previously threatened violence.

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Robert Crimo Jr crying in court

Robert E. Crimo Jr., right, wipes his eye as he listens to Judge George D. Strickland during an appearance at the Lake County Courthouse, Monday, Aug. 7, 2023, in Waukegan, Ill. (AP Photo/Nam Y. Huh, Pool)

Per state law, "a person commits reckless conduct when he or she, by any means lawful or unlawful, recklessly performs an act or acts that... cause great bodily harm or permanent disability or disfigurement to another person."

Crimo Jr.’s lawyers argued that the law makes it impossible to know what actions qualify as criminal conduct. It says the law also offers no definition of "cause," opening the way for prosecutors to link the signing of a gun-license application to a shooting years later.

Highland Park shooting suspect in court

FILE: Robert Crimo III, 21, seen here in lower image at a July 6, 2022, bond hearing, has been charged with seven counts of first-degree murder after allegedly firing into the crowd at a Fourth of July parade in Highland Park, Illinois.  (Antonio Perez/Chicago Tribune/Tribune News Service via Getty Images)

Lake County Judge George Strickland said at Monday’s hearing he would issue a ruling at the next hearing, on Aug. 28. If he rejects Gomez's arguments and allows the case to proceed, Crimo Jr.'s bench trial would start Nov. 6. 

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A grand jury indicted the son last year on 21 first-degree murder counts, 48 counts of attempted murder and 48 counts of aggravated battery, representing the seven people killed and dozens wounded in the July 4, 2022, attack. Potential evidence is voluminous in the son's case and no trial date has been set. He has pleaded not guilty.

Robert Crimo III in court

Bomb-making materials were reportedly found in the apartment of Highland Park, Illinois, parade gunman Robert Crimo III. (AP Photo/Nam Y. Huh, Pool, File)

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Lake County State's Attorney Eric Rinehart said after the father's arrest that the accusations against him are based on sponsorship of his son's application for a gun license in December 2019. Authorities say Crimo III tried to kill himself in April 2019 and in September 2019 was accused by a family member of making threats to "kill everyone."

The Associated Press contributed to this report.