(CNN) Massachusetts Democratic Sen. Elizabeth Warren is questioning the Trump administration's enforcement of steel tariffs, saying that a majority of early waivers granted went to foreign-owned companies.

Companies with operations in the United States, but headquartered in Japan and China, were more likely to have their requests granted than American companies seeking relief, according to a review done by Warren's staff of more than 900 decisions made during the first 30 days that responses were made available.

"I write to express grave concern that the majority of the first batch of exemptions your department has issued from the Trump administration's steel tariffs have gone to subsidiaries of foreign-owned companies — even though these tariffs are purportedly in place to protect American companies," wrote Warren in a letter sent Tuesday to Commerce Secretary Wilbur Ross.

In March, the Trump administration imposed a 25% tariff on steel products imported into the United States. The duties are meant to protect domestic steel producers, which the administration deems crucial to the country's national security.

But the Commerce Department also allows importers to apply for a waiver from those tariffs if they can't find a domestic supplier for the product they need. Only entities located in the United States can apply for exclusions.

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