President Donald Trump accused the Justice Department today of trying to frame him by planting a spy in his 2016 campaign \u2014 an allegation his own lawyer said might not be true. Promoting a theory that is circulating in conservative circles, Trump quoted Fox Business anchor David Asman and tweeted: "Apparently the DOJ put a Spy in the Trump Campaign. This has never been done before and by any means necessary, they are out to frame Donald Trump for crimes he didn't commit." But Trump lawyer Rudy Giuliani cast some doubt on that. On whether there was an "informant" in the 2016 presidential campaign, Giuliani told CNN, "I don't know for sure, nor does the president, if there really was one," though he said they have long been told there was "some kind of infiltration." Last week, the National Review raised the question of a possible FBI spy in Trump's campaign. The article cites work by Republican Rep. Devin Nunes, an ardent Trump supporter and head of the House intelligence committee, who has demanded information on an FBI source in the Russia investigation. Virginia Senator Mark Warner, the top Democrat on the Senate intelligence committee, objected today to such demands, emphasising "the critical importance of protecting sources and methods." "It would be at best irresponsible, and at worst potentially illegal, for members of Congress to use their positions to learn the identity of an FBI source for the purpose of undermining the ongoing investigation into Russian interference in our election," Warner wrote in a statement. "Anyone who is entrusted with our nation's highest secrets should act with the gravity and seriousness of purpose that knowledge deserves." The New York Times reported separately this week that at least one government informant met several times with Carter Page and George Papadopoulos, both former foreign policy advisers for Trump's Republican campaign. The newspaper attributed the information to current and former FBI officials. Also today, Giuliani said special counsel Robert Mueller has narrowed his possible interview subject areas from five to two as negotiations continue over whether the president will sit down and answer questions in the Russia investigation. Mueller is investigating possible coordination between Russia and Trump's 2016 campaign.