That's a very good question; the answer is The Speaker of the House, who was Nancy Pelosi in Mar 2021. Very interesting.

While I was looking around that .gov website I caught this as a responsibility of the SAA

Emergency Preparedness The Office of Security and Emergency Preparedness provides the management structure to oversee and integrate security and emergency preparedness planning, policies, and programs for the Senate. Working in close cooperation with the secretary of the Senate, the House of Representatives, and the Capitol Police, this office is responsible for continuity of operations training and assistance to the Senate as well as integrating Senate security plans.

Continuity of operations, you say ...

That's a very good question; I'll keep looking. While I was looking around that .gov website I caught this as a responsibility of the SAA

Emergency Preparedness The Office of Security and Emergency Preparedness provides the management structure to oversee and integrate security and emergency preparedness planning, policies, and programs for the Senate. Working in close cooperation with the secretary of the Senate, the House of Representatives, and the Capitol Police, this office is responsible for continuity of operations training and assistance to the Senate as well as integrating Senate security plans.

Continuity of operations, you say ...