Its not entirely clear cut. Hoover used the regular Army under Gen Douglas MacArthur with cavalry and six tanks to drive the protesters of the Bonus Army out killing several and burning their tents and possessions.
https://www.globalsecurity.org/military/library/policy/army/fm/19-15/CH1.htm
CHAPTER 1
Civil Disturbances
Civil disturbances arise from acts of civil disobedience. They occur most often when participants in mass acts of civil disobedience become antagonistic toward authority, and authorities must struggle to wrest the initiative from an unruly crowd. In the extreme, civil disturbances include criminal acts of terrorism. Civil disturbances, in any form, are prejudicial to public law and order.
FEDERAL INTERVENTION AND AID
Under the US Constitution and the US Code, the President is empowered to direct federal intervention in civil disturbances to:
Respond to state requests for aid in restoring order.
Enforce the laws of the United States.
Protect the civil rights of citizens.
Protect federal property and functions.
Under the Constitution, each state is responsible for protecting life and property within its boundaries. State and local governments use their civil forces to maintain law and order and to quell civil disturbances. However, if a civil disturbance exceeds the resources of a state, federal troops may be called upon to help restore and maintain law and order.
The Constitution and federal statutes authorize the President to direct the use of federal armed troops within the 50 states, the District of Columbia, Puerto Rico, and US possessions and territories and their political subdivisions. The President is also empowered to federalize the National Guard of any state to suppress rebellion and enforce laws.
Federal aid is given to a state when the state has used all of its resources, including its National Guard, to quell a disorder and finds the resources not sufficient. Usually, active-duty federal forces are used to augment the requesting state's National Guard. But the President may choose to federalize another state's National Guard and use them, alone or with other forces, to restore order.
The President also can employ federal troops to ensure the execution of US law when a state opposes or obstructs US law or impedes the course of justice under those laws. And the President can employ armed federal troops to suppress insurrection, domestic violence, unlawful assemblies, and conspiracy if such acts deprive the people of their constitutional rights and privileges and a state's civil authorities cannot or will not provide adequate protection.
The President is also authorized to use armed federal troops to protect federal property and functions when the need for protection exists and the local civil authorities cannot or will not give adequate protection. The right of the United States to protect all federal property and functions regardless of their locations is an accepted principle of our government.
As a temporary measure, federal military equipment and facilities may be loaned to state and local governmental bodies law and enforcement agencies for use during civil disturbances. These resources may also be loaned to a state's National Guard and to non DOD federal agencies. The requesting agencies are expected to provide enough resources of their own to minimize the need for US military resources. And the loan of the resources must not conflict with US military needs.