Who was the group that attacked the Capitol 20 minutes before Trump supporters got there?

The Director of the FBI testified that the vast majority of the Trump supporters at the Capitol on January 6 were peaceful and law abiding, and that there was only a small group that participated in violence or illegal activities. The Chief of Capitol Police testified that a small group started attacking them 20 minutes before Trump supporters arrived at the Capitol, so who was this small group of violent attackers?

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