"Nearly all [FBI] investigations are assigned to one of the bureau's 56 field offices," according to The Times.
"But given this inquiry's importance, senior F.B.I. officials have opted to keep it closely held in Washington in the agency's counterintelligence section, which investigates how national security secrets are handled."
Though Clinton's use of a private email address was not illegal and was permitted by State Department rules, the federal government has standards for how servers are built, how they are secured, and how their data is stored.
The FBI is looking into the configuration of the server that Clinton handed over to authorities.