WASHINGTON (Reuters) - Ten Senate Democrats complained in a letter to U.S. President Donald Trump on Friday over the lack of transparency in trade negotiations by his administration and called on him to direct agency heads to release documents on trade investigations.

The letter comes a day after the Commerce Department submitted a report to Trump on whether foreign steel imports threaten U.S. national security.

Among the letter’s signers is Ron Wyden of Oregon, the most senior Democrat on the Senate Finance Committee, which oversees trade policy. Others include Ben Cardin of Maryland, Debbie Stabenow of Michigan, Mark Warner of Virginia, Bob Menendez of New Jersey, and Maria Cantwell of Washington.

In particular, the letter complains about the lack of transparency ahead of the first round of talks last week on modifying a five-year-old free trade agreement with South Korea. Both Republican and Democrats have expressed concern over the decision by Trump to amend the pact, known as KORUS, at a time when tensions with North Korea over its nuclear and missiles programs have escalated.

Since taking office in 2017, Trump has pulled the United States out of talks on a 14-nation Asia-Pacific trade pact, started negotiations on a new deal for the North American Free Trade Agreement between the U.S., Mexico and Canada and initiated a review of the 2012 Korea deal.

“The failure of the administration to release several trade-related reports raise serious questions regarding the administration’s commitment to openness with the American public when it comes to trade policy,” the senators wrote in the letter to Trump.

“We urge you to act swiftly to direct agencies to publicly release all completed trade-related reports and to direct USTR, in its negotiations with Korea to comply with transparency and consultation requirements set out in law in 2015,” they said.

Trump launched the “Section 232” steel investigation in April, opening the way for his administration to impose broad tariffs or import quotas on imports of foreign steel.

The investigation was kept under wraps at the Commerce Department, but the agency faced a Jan. 15 statutory deadline to present its report to the White House. A similar national security probe into aluminum imports invoking the same Cold War-era trade law is due on Jan. 22.

While forcing a reduction of excess production in China, which now supplies half the world’s steel, is a key goal of any potential restrictions, broad tariffs would have a bigger impact on steelmakers in Europe, Japan, South Korea and Turkey.