Washington (CNN) Former national security adviser John Bolton has "personal knowledge" of relevant meetings and conversations "that have not yet been discussed in testimonies thus far" as part of the impeachment inquiry into President Donald Trump, but he is still refusing to testify until a federal judge rules in an ongoing legal fight between House committees and the White House, according to his lawyer.

Bolton's lawyer, Charles Cooper, wrote a letter to lawmakers Friday in which he teased the idea that his client could offer new details related to the impeachment probe , as well as additional context about events that have been described in other witness testimony.

Bolton "was personally involved in many of the events, meetings, and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far," the letter reads.

But despite underscoring Bolton's value as a potential witness, Cooper makes clear that his client is unwilling to testify until the court reaches a decision in the legal fight over claims of immunity for White House officials.

Trump's former national security adviser is at the center of several key events related to the investigation. Those include suggestions that he had raised concerns about the President and Ukraine, calling efforts by some top officials to help push for investigations into former Vice President Joe Biden and matters related to the 2016 election a "drug deal," according to testimony last month from former top White House Russia adviser Fiona Hill.

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