6 USC 111 - Executive department; mission

(a) Establishment 
There is established a Department of Homeland Security, as an executive department of the United States within the meaning of title 5.
(b) Mission 

(1) In general 
The primary mission of the Department is to
(A) prevent terrorist attacks within the United States;
(B) reduce the vulnerability of the United States to terrorism;
(C) minimize the damage, and assist in the recovery, from terrorist attacks that do occur within the United States;
(D) carry out all functions of entities transferred to the Department, including by acting as a focal point regarding natural and manmade crises and emergency planning;
(E) ensure that the functions of the agencies and subdivisions within the Department that are not related directly to securing the homeland are not diminished or neglected except by a specific explicit Act of Congress;
(F) ensure that the overall economic security of the United States is not diminished by efforts, activities, and programs aimed at securing the homeland;
(G) ensure that the civil rights and civil liberties of persons are not diminished by efforts, activities, and programs aimed at securing the homeland; and
(H) monitor connections between illegal drug trafficking and terrorism, coordinate efforts to sever such connections, and otherwise contribute to efforts to interdict illegal drug trafficking.
(2) Responsibility for investigating and prosecuting terrorism 
Except as specifically provided by law with respect to entities transferred to the Department under this chapter, primary responsibility for investigating and prosecuting acts of terrorism shall be vested not in the Department, but rather in Federal, State, and local law enforcement agencies with jurisdiction over the acts in question.