The current and former officials have not said whether the investigation has unearthed any evidence of wrongdoing by Trump aides or any other Americans. Trump spokesman Sean Spicer said on the TODAY Show that the president would let the investigation go wherever it leads.

“Of course — absolutely, I think he’s made that clear,” Spicer told Meet the Press moderator Chuck Todd. “He’s not going to shut anything down.”

Former intelligence officials told NBC News that President Trump would technically have the authority to order an end to the investigation, given that the intelligence agencies report directly to him. But it would be politically disastrous for him to do so, they said.

“I remember the last president who ordered a stop to an investigation and it cost him his presidency,” said Raymond Batvinis, a former FBI counter intelligence agent who teaches national security at George Washington University, speaking of Richard Nixon and Watergate.