Bannon was quizzed voluntarily behind closed doors by the House of Representatives Intelligence Committee, the first time he has testified in the probe investigating whether Trump's campaign colluded with Russia in its bid to influence the 2016 US elections.
It was unlikely to be Bannon's last such testimony: the New York Times and the Washington Post reported late Tuesday that Bannon has been subpoenaed by Robert Mueller, the Justice Department special counsel investigating the same issue.
When appearing before the Intelligence Committee, Bannon refused to answer a number of questions, citing "executive privilege" allowing the president to keep information from the public.
"Steve Bannon and his attorney asserted a remarkably broad definition of executive privilege," Representative Jim Himes, a Democratic member of the committee, said on CNN.
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"Now remember, it's the president who has the executive privilege and so they went back, conferred with the White House, and the White House said that anything that happened, any communications that happened while Steve Bannon was in the White House or during the transition, any communications were off limits," Himes said.
The committee itself eventually subpoenaed Bannon for refusing to answer its questions, reports said. Still, he again balked after the White House was consulted, said Representative Adam Schiff, the top Democrat on the committee.
Bannon "got the same instruction back again. Basically, 'we don't care whether it's under compulsory process or a voluntary basis -- we're instructing you, effectively putting in place a gag rule,'" said Schiff.
The unrestricted testimony of Trump's estranged political strategist could be explosive: he had a front-row seat as chief executive of the 2016 election campaign in its final months, and as a top policy advisor in the first seven months of the administration.