31 C.F.R. § 13.5
The Assistant Secretary may decide to utilize, on a reimbursable basis, the services, personnel, equipment, and facilities of State and local governments of the affected metropolitan area desiring to provide protection, or he may utilize the United States Secret Service Uniformed Division, or both. If the United States Secret Service Uniformed Division is utilized to meet all the extraordinary protective need, the governments of the affected metropolitan area will not be reimbursed. If the United States Secret Service Uniformed Division is utilized to meet part of the extraordinary protective needs, the governments of the affected metropolitan area will be reimbursed for that qualifying portion of the protection which is provided by State and local police authorities. If the Assistant Secretary decides to utilize, with their consent, the services, personnel, equipment, and facilities of such State and local governments to meet the extraordinary protective need, he will so notify the government as soon as possible after receipt of a request for protection or an advance notice of a reimbursement request made pursuant to § 13.4 .
31 C.F.R. §13.5