Charles Kupperman, President Donald Trump's former deputy national security adviser, filed a lawsuit Friday asking a federal judge to determine whether he is required to testify in the impeachment inquiry being conducted by the House of Representatives, NBC News reports.

The outcome could affect the ability of other witnesses who worked closely with the president to cooperate with the ongoing investigation. The lawsuit was first reported by The New York Times earlier Friday.

Kupperman is scheduled to testify Monday. The scheduled interview would bring the inquiry closer into the orbit of John Bolton, Trump’s former national security adviser.

House Democrats are conducting an impeachment inquiry that centers on alleged pressure on Ukraine’s president to announce investigations into what has been called a conspiracy theory into the 2016 presidential election as well as into former Vice President Joe Biden and his son.

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