Steve Bannon, ex-chief strategist to US President Donald Trump, has refused to answer a broad range of questions at a committee hearing probing alleged links between the Trump campaign and Russia.
Mr Bannon was called to testify to the House Intelligence Committee about his time working for Mr Trump.
He was questioned behind closed doors on Tuesday in a 10-hour meeting.
Mr Bannon's lawyer relayed questions to the White House, and was told when not to respond, AP news agency reports.
A Democrat on the committee said Mr Bannon was under a White House "gag order".
Representative Adam Schiff said that Mr Bannon's lawyer had cited "executive privilege" - which allows the president to keep information from the public.
He said that Mr Bannon had refused to answer questions related to his time at the White House, or the transition to the new administration.
"Steve Bannon and his attorney asserted a remarkably broad definition of executive privilege," Jim Himes, a Democratic member of the committee, told CNN.
The panel issued a subpoena to compel him to return for a second round. His first appearance was voluntary.
Mr Bannon's representatives have made no comment.
Asked if the White House had told him not to answer certain questions, White House spokeswoman Sarah Sanders said: "No-one has encouraged him to be anything but transparent."
She said Congress had to consult with the White House prior to obtaining confidential material.
Republican member Mike Conaway told the press that Mr Bannon was "not an executive", and the committee was "going to resolve the issues to get the answers''.
Earlier on Tuesday, it was reported that Mr Bannon had been issued with another subpoena, this time from former FBI director Robert Mueller, who is leading the inquiry into alleged Russian collusion during the 2016 election campaign.
Mr Bannon "intends to fully cooperate with Mueller", a source told Axios, a Washington DC political news outlet.
Mr Mueller's grand jury and the Congressional panel are separate investigations.
FBI agents went to Mr Bannon's home in Washington DC last week to serve him with the grand jury subpoena, according to NBC News.
But the agents were unaware that he had retained a lawyer, William Burck, who is also representing two other ex-Trump officials that have been questioned as part of the Mueller probe.
The agents then delivered the order to Mr Burck instead, the networks reports.
Why is Steve Bannon of such interest?
He helped shape Mr Trump's "America First" election campaign message and served as one of his closest aides before being pushed out of his role in August.
He recently fell out with Mr Trump publicly after reportedly making comments about the president and his family in a newly published book, Fire and Fury: Inside the Trump White House, by Michael Wolff.
He was quoted as calling a meeting in June 2016 between Mr Trump's son Donald Jr and a group of Russians "treasonous".
Why was Mr Bannon in Congress?
The House Intelligence Committee is one of four investigations being conducted by Congress into the alleged collusion, with others launched by the Senate Intelligence Committee, the Senate Judiciary Committee and the House Oversight Committee.
How far has the Mueller inquiry progressed?
Four people have been charged:
- Mr Trump's former campaign manager, Paul Manafort, and Rick Gates, a campaign aide, pleaded not guilty to all charges including allegations of money laundering unrelated to the 2016 election
- George Papadopoulos, a former Trump campaign adviser, admitted lying to the FBI about his contacts with Russians
- Michael Flynn, a former national security adviser to the Trump administration, pleaded guilty to lying to the FBI over meetings he had with Russian ambassador Sergei Kislyak