Highland Park, Illinois, banned assault rifles nearly a decade before Monday's mass shooting.
The ban was challenged and the case made its way to the Supreme Court, which refused to hear it.
In a dissent, Justice Clarence Thomas wrote that an "overwhelming majority" use such weapons lawfully.
Nearly a decade before Monday's deadly mass shooting at a Fourth of July parade in Highland Park, Illinois, that left six dead and dozens injured, the Chicago suburb banned assault rifles like AR‐15s and AK‐47s.
That 2013 ban was swiftly challenged and the case made it all the way to the US Supreme Court, which ultimately rejected to hear it and instead let a lower court's ruling in favor of the ordinance stand.
But at the time, conservative Justice Clarence Thomas — who joined the nation's highest court in 1991 — opposed Highland Park's prohibition on assault weapons and large-capacity magazines, writing in a dissent that the "overwhelming majority" who use such weapons use them lawfully.
In Thomas' dissent, which was filed in December 2015 and joined by the late Justice Antonin Scalia, he called assault weapons "modern sporting rifles" and made references to the Second Amendment.
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"The ordinance criminalizes modern sporting rifles (e.g., AR-style semi automatic rifles), which many Americans own for lawful purposes like self-defense, hunting, and target shooting," the dissent read.
Thomas deemed the city's ban "highly suspect because it broadly prohibits common semiautomatic firearms used for lawful purposes."
"Roughly five million Americans own AR-style semiautomatic rifles," read the dissent, which added, "The overwhelming majority of citizens who own and use such rifles do so for lawful purposes, including self-defense and target shooting."
The United States Court of Appeals for the Seventh Circuit upheld Highland Park's assault weapons ban after a local doctor and the Illinois State Rifle Association sued the city over it.
"[A]ssault weapons with large‐capacity magazines can fire more shots, faster, and thus can be more dangerous in aggregate. Why else are they the weapons of choice in mass shootings?" the appellate court wrote in its April 2015 opinion.
The court added, "A ban on assault weapons and large‐capacity magazines might not prevent shootings in Highland Park (where they are already rare), but it may reduce the carnage if a mass shooting occurs."
Authorities recovered a "high-powered rifle" from the scene after Monday's mass shooting in Highland Park, which comes on the heels of massacres in Uvalde, Texas, and Buffalo, New York.
It was not yet clear where the gun came from — but Democratic Highland Park Mayor Nancy Rotering who signed the ordinance banning assault weapons in 2013 — said that the gun used in the shooting was "legally obtained."
A suspect in the shooting was taken into custody on Monday after an hours-long manhunt, the FBI announced.
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