The liberal protester who earlier this week infiltrated a group of reporters in a secure area of the Capitol, tossed mini-Russian flags and shouted "treason" at President Trump had been on a watch list before Tuesday’s incident, Fox News has learned.

The protester was identified as Ryan Clayton, an activist from an anti-Trump group who was later arrested and charged with unlawful conduct.

The U.S. Capitol Police Board is conducting an internal investigation into how someone without a press credential – and on a watch list – was able to slip into the secure area and get so close to the president.

“This was a monumental f---up,” a senior congressional security source told Fox News.

PROTESTER THROWS RUSSIAN FLAGS AT TRUMP, SHOUTS 'TREASON' AFTER BLENDING IN WITH PRESS

Clayton has been on a “watch list” kept by the Capitol Police, a source said, and was denied entry to the complex earlier that day when an officer recognized him at the Russell Senate Office Building.

But later on, officers in a tunnel leading to the Capitol did not check his ID to see if he had a pass or badge, the source said. He used an outdated visitor’s pass to enter the Capitol through a tunnel connecting the Rayburn House Office building. After clearing the security checkpoint, he made his way outside the Senate chamber, where he blended in with credentialed congressional reporters and stood just feet away from Trump.

A source also said Clayton was in possession of a four-day old pass for the location of the House Sensitive Compartmentalized Information Facility, which is home to the House Intelligence Committee. Fox News is told there is no evidence so far that Clayton attempted to access that facility.

The startling scene with the president played out in full view of the cameras as Trump walked down the Capitol halls with Senate Majority Leader Mitch McConnell, R-Ky.

“Trump is treason!” Clayton shouted from an area designated for the press, throwing the Russian flags in the air.

Clayton is known for expressing his anti-Trump views by tossing flags. His group, Americans Take Action, is anti-Trump, with tabs on its website dedicated to his impeachment, “Trump Puppets” and options to “Get Russian Trump Flags.”

At the Conservative Political Action Conference in February, he spearheaded the distribution of over 1,000 Russian flags with Trump's name printed across the middle.

Fox News has also learned he was responsible for stirring a commotion at the Capitol in July during a visit by White House aide Jared Kushner.

He was also one of three disruptive people pulled out of the House chamber in January during the tabulation of the Electoral College during a Joint Session of Congress.

Fox News' Chad Pergram, Brooke Singman, Jason Donner and Ben Florance contributed to this report.