Special counsel Robert Mueller has requested interviews from three current and former intelligence officials, a former senior intelligence official familiar with the discussions confirmed to NBC News on Thursday.
Director of National Intelligence Dan Coats, NSA chief Mike Rogers and Richard Ledgett, former deputy director of the NSA, have agreed to be interviewed, the official told NBC.
The Washington Post first reported the requests.
A spokesman for Trump's lawyer told NBC: "The FBI leak of information regarding the president is outrageous, inexcusable and illegal."
As the probe expands to include the events surrounding Comey's abrupt removal, it's possible Mueller's investigators will request interviews from other government officials.
Earlier this month, Deputy Attorney General Rod Rosenstein told The Associated Press he would recuse himself from overseeing Mueller's investigation if his role in Comey's firing becomes relevant to the inquiry.
"I've talked with Director Mueller about this," Rosenstein told AP. "He's going to make the appropriate decisions, and if anything that I did winds up being relevant to his investigation then, as Director Mueller and I discussed, if there's a need from me to recuse, I will."
Rosenstein wrote the memo laying out the case for new leadership at the FBI. He subsequently appointed Mueller as special counsel for the investigation into Russian efforts to interfere in the 2016 election.
— NBC News and the Associated Press contributed reporting.