Former Secretary of State Rex Tillerson will testify in the trial of a billionaire investor who advised Trump

Tom Barrack's lawyers indicated the Department of Justice called Tillerson as a witness and requested that he testify on Monday.

• Former Secretary of State Rex Tillerson will testify in the trial of billionaire investor Tom Barrack.
• Barrack, who worked with Trump, is being charged with illegal lobbying.
• Prosecutors say Barrack tried to alter Trump speeches to make them more favorable to the UAE.

Former Secretary of State Rex Tillerson is expected to testify in the trial of a former adviser to former President Donald Trump who is charged with colluding with the United Arab Emirates.

In a letter submitted Saturday, billionaire investor Tom Barrack's lawyers indicate the Department of Justice called Tillerson to testify as a witness next week. Barrack's lawyers requested in the letter, reviewed by Insider, that Tillerson be called to the stand on Monday in order to have sufficient time to cross-examine him.

Tillerson served under Trump between 2017 and 2018.

Barrack is being charged with illegally acting as a foreign agent on behalf of the UAE, obstruction of justice, and making false statements to the FBI. Barrack was charged in July of last year and his trial began last week in Brooklyn, New York.

Prosecutors say Barrack attempted to add language favorable to the UAE in Trump's speeches in order to sway officials on policies related to the country.

Barrack informally advised the former president during his campaign and also served as a member of the President's Inaugural Committee under Trump. 

Representatives for Tillerson and the US Attorney's Office in New York did not immediately respond to Insider's request for comment.