The Commerce Department's office of inspector general (IG) is planning to audit the process by which firms gain exemptions to U.S. tariffs, according to documents obtained by The Hill.

According to an internal letter dated Monday, the IG is investigating how decisions are made to assign exemptions for President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's steel and aluminum tariffs.

The IG will review the Bureau of Industry and Security and the International Trade Administration's processes for assigning aluminum and steel tariff exemptions.

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A spokesman for the IG confirmed to The Hill on Thursday that the audit is taking place.

Lawmakers on both sides of the aisle have expressed concerns about the process by which tariff exemptions are adjudicated.

Sen. Ron Johnson Ronald (Ron) Harold Johnson CIA found Putin 'probably directing' campaign against Biden: report This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda GOP set to release controversial Biden report MORE (R-Wis.) sent a letter to Commerce Secretary Wilbur Ross Wilbur Louis RossTrump admin asks Supreme Court to fast-track excluding people in U.S. illegally from census Trump 'very happy' to allow TikTok to operate in US if security concerns resolved TikTok, WeChat to be banned Sunday from US app stores MORE in August demanding he explain how firms are granted exclusions and calling the process "arbitrary."

"The Department's denial of the exclusion request has resulted in [one] Wisconsin business incurring an additional $2.6 million tariff cost that can not be used to expand production or to pay salaries of new employees," Johnson wrote in the letter.

"Across the country, many businesses share the same frustration about the difficult and time-consuming process."

That same month, Sen. Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (D-Mass.) called on the IG to look into the steel and aluminum tariff exemption review process.

"This process appears to be running on an ad hoc basis, with little transparency, and bending to political pressure from well-connected lobbyists and administration officials," she contended.

There have been complaints about the exemption process since the tariffs rolled out, after the Commerce Department was caught in a flood of requests, allegedly leading to disarray, The New York Times reports.