Congressional sources who were present late Monday for a security briefing tell TMZ ... the members were told these extremist groups, including the Proud Boys, have either a loose or no real connection to each other, especially since it's increasingly hard for them to go back and forth with each other on social media because of the various blackouts. Trump, they were told, is the glue that will hold these groups together once they assemble in D.C.

The Capitol Police and the Sergeant at Arms ran the meeting, and told members that security at the Inauguration will include metal detectors and even members of Congress must pass through. The officials running the meeting said they can't take anything for granted, especially given that there seems to be a small QAnon faction that presents a danger.

The meeting was frightening ... there was talk that the extremists might target Democrats for murder so the Republicans could take over Congress.

Our sources say the members have all been told they can get security for themselves if there is a "direct threat" to them, and that's already the case with many members of Congress.

There's something else ... D.C. Mayor Muriel Bowser has urged the public to stay away from the Inauguration. As a result, the members were informed there's an assumption the majority of the crowd could be comprised of these extremists. The silver lining ... they will be easier to ID. As one member told TMZ, "at least we will know who they are."