When does the FBI get involved and why?

[Kimster]

Many of us get curious when the FBI is called in. Others are upset when they aren't.

Here we can discuss what causes FBI involvement.

 

[Patience]

What the FBI investigates- Copied from the FBI webiste

http://www.fbi.gov/about-us/investigate/what_we_investigate
Spies. Terrorists. Hackers. Pedophiles. Mobsters. Gang leaders and serial killers. We investigate them all, and many more besides.
The very heart of FBI operations lies in our investigations—which serve, as our mission states, "to protect and defend the United States against terrorist and foreign intelligence threats and to enforce the criminal laws of the United States." We currently have jurisdiction over violations of more than 200 categories of federal law, and you can find the major ones below, grouped within our three national security priorities and our five criminal priorities. Also visit our Intelligence program site, which underpins and informs all our investigative programs.

 

[Liz81]

Sorry, I know this is a very old thread but found it when doing some googling. What if the police decided to stay silent like about a serial criminal? Does the FBI still get involved anyway? Doesn't the police actually call the FBI when they're necessary?