I believe FBI can be called in at anytime by local law enforcement.
From their website:
State and local law enforcement agencies are not subordinate to the FBI, and the FBI does not supervise or usurp their investigations. However, through cooperation, the investigative resources of the FBI and state and local agencies often are pooled in a common effort to investigate and solve the cases.
Also they have a special CAC program:
The mission of our Crimes Against Children (CAC) program is threefold: first, to decrease the vulnerability of children to sexual exploitation; second, to develop a nationwide capacity to provide a rapid, effective, and measured investigative response to crimes against children; and third, to enhance the capabilities of state and local law enforcement investigators through programs, investigative assistance, and task force operations. Our strategy involves using multi-disciplinary and multi-agency teams to investigate and prosecute crimes that cross legal, geographical, and jurisdictional boundaries; promoting and enhancing interagency sharing of intelligence, specialized skills, and services; and widely offering our victim/witness services.