The families of nine of the victims and one survivor have said Remington, along with a gun wholesaler and local retailer, are partially responsible for the carnage at Sandy Hook Elementary School in Newtown, Connecticut, because they marketed the weapon based on its militaristic appeal.
The plaintiffs alleged that Remington promoted the Bushmaster AR-15 style rifle for use against "perceived enemies", an argument a majority of the judges said deserved further scrutiny. He then committed suicide.
"The number of lives lost in those 264 seconds was made possible by the shooter's weapon of choice", the plaintiffs said in their lawsuit.
Lanza, 20, shot his way into the locked school in Newtown on December 14, 2012, and killed 20 first-graders and six educators with a Bushmaster XM15-E2S rifle, similar to an AR-15.
The 2005 Protection of Lawful Commerce in Arms Act, or PLCAA, has so far shielded the USA firearms industry from lawsuits by victims of gun violence, including major players such as Remington, Sturm Ruger and Co and Vista Outdoor Inc from bearing any liability.
The decision was hailed by the lawyers for the plaintiffs as "a crucial step" towards exposing "Remington's calculated and profit-driven strategy to expend the AR-15 market...at the expense of Americans' safety".
Lanza, 20, used the rifle to murder 20 children aged between six and seven, along with six adult members of staff at Sandy Hook Elementary School in December 2012, before turning the gun on himself. It also concluded that CUPTA lawsuits are not barred by a federal law that generally protects gun suppliers from civil liability for crimes committed with their products.More news: Shooting at a mosque in New Zealand's Christchurch; casualties feared
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The families argued that the manufacturer, distributor and seller of the weapon negligently entrusted to civilian consumers an assault rifle that is suitable for use only by military and law enforcement personnel and violated the Connecticut Unfair Trade Practices Act (CUTPA) through the sale or wrongful marketing of the rifle.
The ruling on Thursday overturned a lower court's judgment rejecting the lawsuit, which charged that Remington Outdoor Co., arms distributor Camfour, and the CT store which sold the gun used in the massacre could be held liable.
The group Connecticut Against Gun Violence said the state supreme court decision "will force the companies to reveal internal communications that they have fought to keep out of the public eye".
Military-style rifles have been used in many other mass shootings, including in Las Vegas in October 2017 when 58 people were killed and hundreds more injured.
The case had been closely watched by advocates on both sides of the gun issue.
The 2005 federal law has been cited by other courts that rejected lawsuits against gun makers and dealers in other high-profile shooting attacks, including the 2012 Colorado movie theater shooting and the Washington, D.C., sniper shootings in 2002.
Still, allowing the lawsuit to move forward means that there will be an opportunity for discovery that would unearth company documents that could be embarrassing for Remington.