Originally Published: 24 JUN 22 15:00 ET

By Katelyn Polantz, Andrew Millman and Holmes Lybrand, CNN

(CNN) -- Ali Alexander, the leader of the "Stop the Steal" group who helped to organize rallies before the US Capitol attack, testified to a federal grand jury on Friday in the Justice Department's sweeping investigation into the January 6, 2021, insurrection.

CNN spotted Alexander entering and exiting the federal courthouse in Washington, DC, where a federal grand jury that has heard January 6-related matters meets. He spent about four hours at the courthouse on Friday behind closed doors.

Alexander did not respond to questions at the courthouse. He has not been charged with any crime.

He acknowledged his testimony in a social media post on Friday. "Today a DC grand jury brought me in under subpoena and the threat of imprisonment. I told them I committed no crimes and observed no one committing any crimes during the 2020 Election and January 6th," he wrote on the Trump-founded site Truth Social.

Alexander is the first-known, high-profile witness to testify in the confidential criminal investigative proceedings related to rally organizing. In April, he publicly confirmed he had received a subpoena and would cooperate with federal authorities.

He previously handed over thousands of communications and testified behind closed doors to the House select committee investigating the attack -- a separate investigation. Alexander is one of the few individuals who has said he has ties to influential right-wing figures close to former President Donald Trump, members of Congress and extremist groups.

CNN previously unearthed videos of Alexander saying he would reach out to the Proud Boys and Oath Keepers to provide security for his events in DC. Leaders from both groups have been charged with seditious conspiracy and are headed to trial later this year.

This story has been updated with further developments Friday.