As President Donald Trump pressures Mexico to address the flow of migrants and drugs heading north into the United States, the Supreme Court is set to hear arguments Tuesday in a major appeal about one thing that’s crossing the border toward Mexico: guns.

Justices expressed deep skepticism Tuesday about an unusual lawsuit by the Mexican government that seeks to hold major U.S. firearms manufacturers accountable for gun violence in that country.

Do U.S. arms manufacturers bear responsibility for the guns fueling cartel violence in Mexico? Mexico is arguing that case before the Supreme Court.