Transcript for Investigation: How Did Clinton Donor Get on National Security Board?

This comes amid an ABC news investigation about a donor to the Clinton campaign. He was named to a board while Hillary Clinton was secretary of state. Here's Brian Ross. Reporter: It was her promise, under oath at her confirmation hearing for secretary of state, that there would be no special treatment for donors to the Clinton foundation. That will not influence, it was not be in the atmosphere. Reporter: Yet, ABC news discovered one of the foundation's big donors was appointed by secretary Clinton in 2011 to a state department advisory board that deals with top secret American nuclear strategy, even though the donor, rajiv Fernando, a Chicago commodities trader, had no known expertise in that area. I'd like to invite to the stage, Raj Fernando. Reporter: What Fernando was known for, before and after the appointment, was raising and giving hundreds of thousands of dollars to democratic campaigns, and as much as $250,000 to the Clinton foundation. ABC news. Do you have a second to talk here? When we approached Fernando at the 2012 democratic convention -- how did you get appointed to that board? How did you know my name, out of curiousty? Reporter: He became upset and we were threatened with arrest. Leave or you will be arrested. Reporter: You are going to arrest me for asking a question? But now a new set of state department e-mails, obtained after a lawsuit brought by citizens united, reveals the role of Hillary Clinton's chief of staff, Cheryl mills in appointing the big donor. In this e-mail, a senior state official, uses the shorthand "S," a common reference to secretary Clinton. The true answer is simply that "S" staff, Cheryl mills, added him. Raj was not on the list sent to "S." He was added at their insistence. The e-mails also show that when ABC began asking questions in 2011 about the appointment of the donor, mills asked that the press officer stall for 24 hours. The very next day, rajiv Fernando submitted his resignation from the sensitive government board. Today, Donald Trump attacked Hillary Clinton on this report. They all looked, where did this guy come from? Reporter: The Clinton campaign declined to comment on our story. As for Fernando, he has continued to raise big money for the Clinton campaign and give millions to the Clinton foundation. In fact, he will be one of her superdelegates at next month's convention. Brian, thank you. Next tonight to new details

This transcript has been automatically generated and may not be 100% accurate.