Former national security adviser John Bolton has information about “many relevant meetings and conversations” related to Ukraine that House impeachment investigators have not yet heard testimony about.

That’s according to a letter that Bolton’s attorney, Charles Cooper, sent Friday to the House that suggests Bolton would appear in the probe only if a court orders him to do so.

In the letter, Cooper says there’s a tall barrier to forcing Bolton and his former deputy, Charles Kupperman, to testify because any testimony that they would give would implicate sensitive matters of national security and foreign affairs. Kupperman has sued to request a judge’s guidance on whether he can be forced to appear.

The letter says both Bolton and Kupperman are prepared to appear if a federal judge resolved the dispute in Congress’s favor.