The FBI investigates crimes inside the U.S. and the CIA investigates crimes outside the U.S.

So, anything criminal at a Federal level can at times be investigated by the FBI whereas the CIA investigates crimes against America outside the U.S. However, it is important to understand that the CIA is primarily an International  Spy agency in favor of the U.S. and the FBI is primarily an investigatory agency so sometimes even overseas the FBI investigators are sent overseas to figure things out. And to find out the truth of what happened in almost any country on earth regarding Americans.

If you are interested in the first CIA agent to die in the line of duty it is a very interesting story told well in "Into Tibet". He was doing research on Nuclear tests in China and Russia while he was in a remote location in China near Tibet. However, when he crossed over into Tibet the people sent to meet him were very ignorant and when they were told to bring the American CIA agent to the Dalai Lama, they thought they could just bring his head which is what they did. The Tibetan government was really angry at this because they wanted the man not just his head. But, strangely enough this was the first CIA agent to be accidentally killed. It makes you think the man who beheaded him was being paid off By China to kill him for researching into nuclear blasts in China and Russia.

Into Tibet: The CIA's First Atomic Spy and His Secret ... - Amazon.com

https://www.amazon.com/Into-Tibet-Atomic-Secret-Expedition/dp/080213999X

Into Tibet: The CIA's First Atomic Spy and His Secret Expedition to Lhasa [Thomas Laird] on Amazon.com. *FREE* shipping on qualifying offers. Into Tibet is the ...

Into Tibet | Grove Atlantic

https://groveatlantic.com/book/into-tibet/

"A scrupulously documented account of Cold War intrigue. . . . [Provides] a detailed view into the CIA's shadowy world and the havoc it wreaks on individual.

Into Tibet: The CIA's First Atomic Spy and His Secret ... - Foreign Affairs

https://www.foreignaffairs.com/.../tibet-cias-first-atomic-spy-and-his-secret-expedition...

Laird has gone to great lengths to penetrate the walls of secrecy the CIA constructed to hide the identity and the activities of Douglas Mackiernan, the first of its ...

Into Tibet: The CIA's First Atomic Spy and His Secret ... - Goodreads

https://www.goodreads.com/book/show/705227.Into_Tibet

 Rating: 3.6 - ‎48 votes

Into Tibet is the incredible story of a 1949-1950 American undercover expedition led by America's first atomic agent, Douglas S. Mackiernan -- a covert attempt to ...

INTO TIBET: THE CIA'S FIRST ATOMIC SPY | CIA FOIA (foia.cia.gov)

https://www.cia.gov/library/readingroom/document/0005543224

INTO TIBET: THE CIA'S FIRST ATOMIC SPY. Collection: FOIA Collection · Declassified Articles from Studies in Intelligence: The IC's Journal for the Intelligence ...

Douglas Mackiernan's - Into Tibet - The CIA's First Atomic Spy and his ...

www.intotibet.info/aboutthebook.html

For over fifty years the Central Intelligence Agency has closely guarded the identity of the first agent killed in the line of duty–the unnamed First Star on the outfit's ...