(CNN)A Pennsylvania state appeals court has decided that the Protection of Lawful Commerce in Arms Act of 2005 is unconstitutional, court documents filed on Monday show.
The PLCAA is a federal law that prohibits civil lawsuits from being brought against gun manufacturers and distributors as a result of misuse of their products by others.
The law says businesses should not "be liable for the harm caused by those who criminally or unlawfully misuse firearm products or ammunition products that function as designed and intended."
But the PLCAA violates the 10th Amendment, Pennsylvania Superior Court Judge Deborah Kunselman said, which guarantees states' rights -- essentially stating that matters which are not delegated to the federal government by the US Constitution and are not Constitutionally prohibited, fall under the authority of the state government.
"The only portions of the PLCAA that do not offend the Constitution are its findings and purposes ... and a few definitions," Kunselman wrote.
The 63-page decision also took issue with arguments that the law falls under Congress' power to regulate interstate commerce.
"We hold that merely because, at some point in time, that gun passed through interstate commerce, does not give Congress perpetual authority to regulate any harm it may cause," Kunselman wrote.
The case that took this to court
The decision came down as part of a case against gun manufacturer Springfield Armory, Inc. and firearms store Saloom Department Store. In March 2016, a 14-year-old unintentionally shot and killed his friend J.R. Gustafson in Westmoreland County, Pennsylvania, with a semiautomatic handgun that he believed was unloaded.
Gustafson's parents sued both Springfield Armory and Saloom Department Store, asserting that they "were negligent and strictly liable for manufacturing and/or selling the defective handgun that caused their son's death," court documents state, alleging the gun's design was defective because it lacked a safety feature "to disable it from firing without the clip attached."
In January, a trial judge in Westmoreland County dismissed the lawsuit after ruling that the PLCAA gave immunity to both the gun manufacturer and firearms store.
The Gustafsons appealed. Following Monday's court decision, the case will be sent back to the trial court.
CNN has reached out to attorneys for Springfield Armory, Inc. and Saloom Department Store for comment on Monday's court decision. They previously attempted to have the case thrown out and argued the PLCAA protected them from being held civilly liable for the child's death, court documents show.
Lead attorney calls decision a win for Gustafson's parents
The Gustafsons' lead attorney, Jonathan Lowy, is also Vice President of the Brady Campaign, a nonprofit organization that advocates for gun control and against gun violence.
Lowy called Monday's decision a "win for the parents of J.R. Gustafson and other victims of gun industry negligence and misconduct, who will now have their day in court."
"In finding that PLCAA is unconstitutional in its entirety, the Superior Court of Pennsylvania properly recognizes that states have the authority to hold negligent gun makers accountable in court, and to ensure that all victims have the right to seek civil justice against wrongdoers who cause them harm," Lowy said in a statement.
In a news release Monday, the Brady Campaign said the Pennsylvania superior court is the first appeals court to hold PLCAA unconstitutional in its entirety.
"This access-to-justice victory for gun violence victims shows why it matters who sits on our nation's court," Brady Campaign President Kris Brown said in a statement.
The next court date for the case has been scheduled for October 28, online court records show.
Correction: An earlier version of this story named the wrong official who had ruled the Protection of Lawful Commerce in Arms Act of 2005 is unconstitutional. It was Pennsylvania Superior Court Judge Deborah Kunselman.