Murder charges brought against the father of a US school shooter have laid down a new marker on the issue of parental responsibility.

Colin Gray bought his son Colt an AR-style rifle for Christmas last year, even though the boy had been questioned by police just seven months earlier about online threats to commit a school shooting.

Investigators suspect the 14-year-old may have used that same weapon on Wednesday when he shot dead four people and wounded nine others at Apalachee High School in Winder, Georgia.

The teen has since been charged with murder and – in an unprecedented move – so too has his dad.

Mr Gray, 54, faces two counts of second-degree murder, four counts of involuntary manslaughter and eight counts of cruelty to children.

Together, the charges carry a maximum penalty of 180 years in prison.

The two murder charges against Mr Gray apply to the two teenagers killed in Wednesday’s rampage: Christian Angulo and Mason Schermerhorn, both 14.

Two Apalachee teachers – Richard Aspinwall, 39, and Cristina Irimie, 53 – also died in the attack.

The second-degree murder charges now facing Mr Gray may be due to specific wording in Georgia law.

According to the state’s criminal code, a person commits second-degree murder “when, in the commission of cruelty to children in the second degree, he or she causes the death of another human being irrespective of malice”.

With prosecutors bringing these charges barely more than 24 hours after the shooting, experts caution that the facts are still emerging, and it remains unclear what legal arguments will be directed at Mr Gray.

“There’s a connection between the deaths and ‘the commission of cruelty to children,'” said Evan Bernick, an associate law professor at Northern Illinois University.

“But is the cruelty directly arising from the shooting, or is it cruelty to his son that may have led [the boy] to commit the shooting? We just don’t know yet.”

The son will be tried as an adult, meaning that the criminal justice system will treat his homicide prosecution as that of somebody fully responsible for their own actions.

But that does not mean his father will escape punishment, Prof Bernick told the BBC.

The crux of the argument will be not that Colin Gray wanted the shooting to happen, but that he “failed to intervene, and his failure to intervene was negligent in ways that justify treating him as part of the homicide”.

Across the US, there are laws on the books to punish parents or guardians for everything from academic truancy and underage driving to shoplifting and vandalism.

But prosecutors in the state of Michigan expanded the reach of such statutes earlier this year when they secured dual convictions against the parents of another teen gunman.

James and Jennifer Crumbley were found guilty of involuntary manslaughter and sentenced to at least 10 years in prison for how their criminal negligence as parents contributed to their son Ethan, 14, killing four of his classmates in 2018.

Thursday’s decision to charge the father with murder – a far more severe charge – could again test the legal bounds of parental responsibility.

Eve Brank, a psychology professor at the University of Nebraska-Lincoln, researches how the law intervenes and sometimes interferes with family decision-making.

In her view, the emerging concept of punishing parents after school shootings reflects broader frustration around US gun violence and, in the absence of regulatory reform, the inability to curb the country’s unrelenting series of firearm incidents.

“It’s not like we’ve created a bunch of new laws to address these issues. They’re just being used, somewhat creatively, to address the issue,” she said.

“In terms of what the research shows, most people would agree there are a lot of influences on how children behave, not just their parents.”

But she noted that prosecutors in Georgia may be privy to information from the investigation not yet publicly available and may believe they can successfully argue that, like the Crumbleys before him, Colin Gray’s actions were particularly egregious.

Tim Carey, a law and policy adviser at the Johns Hopkins Center for Gun Violence Solutions, argues that charging parents is also a reflection on weak gun safety policies.

Georgia has been “very apprehensive to gun violence prevention policies”, he said, and prosecutors in such states may “feel confined to trying to bring a sense of justice or retribution after the fact, in part because they couldn’t prevent” such a tragedy.

Some legal scholars worry that expanding the toolkit prosecutors can use after a shooting could have unintended consequences.

“We know we have a problem of violence and guns in our society,” said Ekow Yankah, a law and philosophy professor at the University of Michigan.

“And instead of tackling it with systemic and regulatory powers, we soothe ourselves with these kind of extraordinary prosecutions.”

But, Prof Yankah warns, prosecutors are now armed with “a hammer” they can bring down on others, including poor families from minority groups and single parents.

“School shootings are highly visible… but I’m worried about the cases that won’t make the news,” he said.

And while parents are now at greater risk of being penalised for their children’s violent actions, less progress has been made on the widespread access to firearms or on the availability of mental health resources for struggling kids.

“Our default response to very deep social problems in the United States is to bring in the apparatus of the criminal law,” said Prof Bernick.