The name of this Fold3.com record collection is something of a misnomer, since the federal agency that would one day be known as the FBI (Federal Bureau of Investigation) was at the time these case files were created known simply as the Bureau of Investigation. These case files include investigative reports and correspondence concerning acts of espionage and sabotage during World War I; German aliens in the U.S. whose loyalty was suspect; Mexican neutrality violations and gunrunning allegations; and violations (and alleged violations) of various Federal laws. Investigative reports and correspondence include items written by Bureau of Investigation agents, other law enforcement officials, and informants.
The FBI Case Files consist of the following record series:
Bureau Section Files, 1909-1921
Mexican Files, 1909-1921
Miscellaneous Files, 1909-1921
Old German Files, 1909-1921
The largest record series by far is the Old German Files. If you are researching any German-American ancestors who may not have naturalized prior to World War I, this record collection may well contain much information of interest.