President Trump on Tuesday signed an executive order directing the Commerce Department to open a national security investigation that would lay the groundwork for the administration to impose tariffs on critical minerals, the latest sector to potentially face tariffs as part of Trump’s growing trade war.

Trump signed an order initiating the probe, known as a Section 232 investigation, that will determine the impact on national security of imported processed critical minerals. 

The executive order makes the case that importing critical minerals has led to “significant” dependence on foreign countries for those materials.

The dependence of the United States on imports and the vulnerability of our supply chains raises the potential for risks to national security, defense readiness, price stability, and economic prosperity and resilience,” the order states.

Critical minerals and rare earth elements are a specific group of materials that are only deposited in relatively small quantities and can be used in a variety of applications, including electronics, health care, and batteries.

Since they are naturally occurring, it may be more difficult for the U.S. to produce more processed critical minerals on its own. Last month, the president signed an order invoking wartime powers under the Defense Production Act to expand domestic U.S. mining production, according to information shared by a White House official. 

Trump on Monday launched a similar probe focused on pharmaceuticals and semiconductors, a sign that tariffs on those sectors could be coming soon as well.

The president has already announced tariffs on automobiles and steel and aluminum, as well as a baseline 10 percent tariff on all foreign imports. He had announced steeper “reciprocal” tariffs on dozens of other countries, but has since lowered those to 10 percent, except for China, which faces a 125 percent reciprocal tariff.

The wave of tariffs, as well as the shifting timelines and exemptions, has caused the stock market to dip. Experts have long warned that companies may pass the cost of tariffs onto consumers, leading to a spike in prices.