The Federal Bureau of Investigation (FBI) is the primary federal law enforcement agency.
The mission of the FBI is to protect and defend the United States against terrorist and foreign intelligence threats, to uphold and enforce the criminal laws of the United States, and to provide leadership and criminal justice services to federal, state, municipal, and international agencies and partners.
FBI Mission Statement
Serving under the Justice Department, the FBI is more akin to policing than the Department of Homeland Security (DHS).
The DHS is more limited in their scope, primarily focused on preventing terrorism.
The vision of homeland security is to ensure a homeland that is safe, secure, and resilient against terrorism and other hazards.
DHS.gov Mission statement
Though their missions seem to both include terrorism within their scope, FBI is a broader federal investigative branch, whose function is more comparable to roles of local police.
I am not sure how to parse out the size of these organizations (or more accurately the number of agents that serve as federal policing agents) because the DHS includes organizations like the Transportation Security Administration (TSA) and other agencies that - I think it is fair to say - are differentiate from policing. However the DHS has agents that function in a law enforcement capacity including the FPS as panda points out - though it should be noted that the FPS is specifically allocated to protecting government facilities and wouldn't be available for more general policing. Like the TSA and FPS,the agencies of DHS seem to specifically designed to provide policing coverage where jurisdictions issues may have complicated the involvement of local police and FBI responsibility.
I believe the National Guard (as a division of the US military) requires the president, an appointee, or a governor to declare a state of emergence to deploy them. In issues of security without an active threat, this may fall outside of their need, and would not be an appropriate situation to call upon the National Guard. You may be comparing this event, to events like the riots that occurred over the summer of 2016 or the LA riots in 1992. Those cases were examples of escalated incidents where a state of emergency is declared. Charlotte Protest Turns Violent, Governor Declares State of Emergency and Deploys National Guard