WASHINGTON, Dec 17 (Reuters) - Longtime Donald Trump adviser Roger Stone appeared before a congressional committee investigating the Jan. 6 attack on the Capitol on Friday but said he refused to answer questions. Speaking to reporters after his closed-door appearance, Stone said he invoked his constitutional protection against self-incrimination in response to a subpoena from the House of Representatives Select Committee. Longtime Donald Trump advisor Roger Stone speaks to the media after exiting the O"Neill House Office Building where he was deposed by the House Select Committee investigating the January 6 attack on the U.S. Capitol in Washington, U.S., December 17, 2021. REUTERS/Ken Cedeno Former U.S. President Donald Trump campaign advisor Roger Stone arrives at the O"Neill House Office Building to be deposed by the House Select Committee investigating the January 6 attack on the U.S. Capitol, in Washington, U.S., December 17, 2021. REUTERS/Ken Cedeno Longtime Donald Trump advisor Roger Stone speaks to the media after exiting the O"Neill House Office Building where he was deposed by the House Select Committee investigating the January 6 attack on the U.S. Capitol in Washington, U.S., December 17, 2021. REUTERS/Ken Cedeno 1/2 Former U.S. President Donald Trump campaign advisor Roger Stone arrives at the O"Neill House Office Building to be deposed by the House Select Committee investigating the January 6 attack on the U.S. Capitol, in Washington, U.S., December 17, 2021. REUTERS/Ken Cedeno The Fifth Amendment of the U.S. Constitution protects a person from being forced to disclose information to investigators that might subject them to criminal prosecution. Stone appeared at pro-Trump rallies in Washington on Jan. 5, the day before the attack, and was protected by members of the Oath Keepers, a far-right anti-government group, on that day and the day of the attack. Several members of that group face criminal charges related to the attack. At least two other high-profile witnesses in the investigation have said they would invoke that protection: John Eastman, a former lawyer for then-President Trump, and Jeffrey Clark, a former top Justice Department official during the Trump administration.