When does the FBI investigate kidnappings?

Answer 1

Murder is not a federal crime, and therefore, a death suspected to be a murder would not ordinarily be investigated by any federal agent, absent some special jurisdiction to do so, however, there are many circumstances under which such jurisdiction would arise. The most obvious would be if the death/murder occurred on US government property. Deaths on military bases would generally be investigated by the respective military branch's similar investigative branch (ie: NCIS in the Navy, etc.). The FBI could also take jurisdiction if the death involved a US Government official, theoretically right down to your USPS letter carrier. The FBI can also claim jurisdiction in an investigation if there is suspicion that there has been a violation of another federal law at the base of the crime, most usually the violation of the victim's civil rights. Such is usually the case when the murder is suspected to be racially motivated, part of an organized crime syndicate's activities ("RICO"), etc. Kidnapping across state lines is a federal crime, as is narcotic trafficking, trafficking in prostitution. Also, the FBI makes itself available to assist local and state authorities in murder investigations upon request when it's superior investigative resources (laboratory analysis, DNA, etc) can be of vital assistance. Lastly, in a way that sometimes blurs jurisdictional distinctions, the FBI also have a tendency to get involved when the victim is from a prominent, politically connected family.

Answer 2

Typically once the kidnapper crosses state lines.

Answer 3

The Federal Bureau of Investigation investigates all serial murderers because most of them cross state lines.