WASHINGTON (AP) — The Justice Department has for the first time charged China-based companies and employees with supplying chemicals needed to make fentanyl, a primary driver of an overdose crisis gripping the United States, top officials said Friday.

The companies openly advertised the chemicals on social media platforms and shipped them surreptitiously to buyers, even adding molecules to evade testing, said Attorney General Merrick Garland.

“We are targeting every step of the movement and manufacturing sale of fentanyl, from start to finish,” Garland said.

Two of the eight people charged have been arrested in the Drug Enforcement Administration investigation into four companies, which also seized more than 200 kilograms of precursor chemicals needed to make fentanyl. The synthetic opioid is considered exponentially more addictive than heroin and is now the leading cause of death for Americans ages 18 to 49.

They gave customers blueprints for making fentanyl and targeted advertising to Mexico, where the cartels that make most of the fentanyl that ends up in the U.S. are based, said DEA administrator Anne Milgram.

The charges include fentanyl trafficking, precursor chemical importation and money laundering.

One company, Amarvel Biotech, posted on its website about shipping to Culiacan, Mexico, the home city of the Sinaloa Cartel, which was the subject of a sweeping prosecution filed two months ago, authorities said. The company did not immediately respond to an email seeking comment.

Deputy Attorney General Lisa Monaco called on the Chinese government to hold accountable companies there that sell the chemicals needed to make fentanyl.

“This is a global problem that demands a global solution,” she said.