The letter sent to Mr. Stone, a copy of which was obtained by The New York Times, represents the first public indication of the scope of the committee’s inquiry, and possible connections to Mr. Trump’s campaign.

The Senate committee asked Mr. Stone, who is also under scrutiny from other federal investigators, to “preserve and retain all hard copies and electronically stored information as specified below in furtherance of the committee’s ongoing investigation into Russian actions targeting the 2016 U.S. elections and democratic processes globally.”

Mr. Stone confirmed the existence of the letter, which was dated Feb. 17. However, he said he had received it only on Friday, by email. Mr. Stone has acknowledged trading messages over Twitter with Guccifer 2.0, the online persona that officials believe was actually Russian intelligence officers…

“I am anxious to rebut allegations that I had any improper or nefarious contact with any agent of the Russian state based on facts, not misleading and salacious headlines,” he said, adding, “I am willing to appear voluntarily if the committee isn’t looking for the headline of issuing a subpoena.”