The Gang of Eight, who have routine access to highly classified materials, include House of Representatives Speaker Paul Ryan, Minority Leader Nancy Pelosi, Intelligence Committee Chairman Devin Nunes and its top Democrat, Adam Schiff.
Senate members include Majority Leader Mitch McConnell, Minority Leader Chuck Schumer, and the top Republican and Democrat on the intelligence committee, Senators Richard Burr and Mark Warner.
A law enforcement source said the Federal Bureau of Investigation was in discussions with the National Security Division of the Justice Department as to how to respond to public and congressional inquiries about the existence or non-existence of such eavesdropping.
If Trump’s campaign or advisers were indeed being wiretapped, the most likely legal path for the Obama administration to do so would be to have the Justice Department ask the Foreign Intelligence Surveillance Court for permission to eavesdrop.
Trump accused predecessor Barack Obama on Saturday of wiretapping him during the late stages of the campaign, but offered no evidence for an allegation which an Obama spokesman said was “simply false”.
The intelligence committee’s letter, addressed to Dana Boente, the acting Deputy U.S. Attorney General, also asks for copies of any such orders actually issued by the court and any electronic surveillance warrants related to Trump or his associates issued last year by a federal judge or magistrate under a wide-ranging anti-crime law.
Reuters saw a copy of the letter, signed by Nunes and Schiff, on Thursday. The letter has not been publicly released.
Law enforcement sources have said that the FBI is pursuing a wide-ranging counter-intelligence investigation of alleged contacts between Trump associates and Russians, as well as two separate investigations into pre-election email hacking linked to Russia which mainly targeted Democratic political operatives.
(Reporting by Mark Hosenball; Additional reporting by Patricia Zengerle; Editing by Alistair Bell and James Dalgleish)