The First Circuit appeared open to Mexico’s argument Monday that gunmakers knowingly manufacture and sell weapons that can be easily modified south of the border by criminal cartels for automatic fire, but were skeptical manufacturers could be held liable.

Mexico alleges that firearm giants, including Smith & Wesson Brands Inc. and Glock Inc., unlawfully design their weapons to attract cartels, exacerbating gun violence in the country. Its novel lawsuit argues that the Protection of Lawful Commerce in Arms Act, which protects gunmakers from an array of lawsuits, shouldn’t apply for claims rooted in foreign law.

If the US Court of Appeals for the First Circuit sides with Mexico, it could open the door to more litigation from other countries against American gunmakers.

However, judges were skeptical that gunmakers should be held liable for weapons that were later modified abroad. Rather than focusing on Mexico’s broader argument about the legal shield, the judges bored in on the question of whether an exception for knowing violations of state or federal laws is applicable here.

That exception would give Mexico an avenue to pursue its claims even if the court finds federal immunity law applies.

“If you violate federal gun laws you do not get protected” by the liability shield, said Jonathan Lowy, president of Global Action on Gun Violence, who represents Mexico.

Judge William Kayatta asked Lowy, “what then is the cause of action that would involve the manufacturer knowingly violating” the law. Lowy responded that the cause of action is the manufacturers allegedly aiding and abetting unlicensed weapons, as well as allegedly designing semi-automatic weapons that can be easily modified to shoot automatically.

“If it was not originally designed as a machine gun, it’s not a machine gun,” said Noel Francisco, a partner at Jones Day representing the manufacturers. Francisco served as US Solicitor General in the Trump administration.

Embracing Mexico’s view of convertible semi-automatic weapons would make the federal government “derelict in its duty” to regulate machine guns in the US, Francisco said.

The manufacturers argue that Mexico hasn’t adequately alleged any violation of federal or state law, and that the “chain of causation” from the manufactures to the injury Mexico seeks relief for is too long to show proximate cause, since the guns are ultimately distributed by retailers that are not named defendants in the lawsuit.

Kayatta suggested that Mexico’s complaint did allege violations of US law.

Judge Gustavo Gelpí also asked both sides whether the lawsuit implicated the Second Amendment.

“You’re talking about a lawsuit that has potentially crippling effects on the ability of an individual firearms owner to obtain firearms,” Francisco answered, arguing that a win for Mexico could allow other countries to govern US firearm policy.

“I don’t think any court has held that there is some Second Amendment right to negligently or illegally hold and make guns,” Lowy said.