The 10-year limit was one provision in the Omnibus Crime Control and Safe Streets Act of 1968. The FBI itself acknowledges that the law was passed "in reaction to the extraordinary 48-year term of J. Edgar Hoover."
Congress passed the law on Oct. 15, 1976, in an attempt to "safeguard against improper political influence and abuses," as Republican U.S. Sen. Chuck Grassley once stated.
It reads, in part:
"Effective with respect to an individual appointment by the President, by and with the advice and consent of the Senate, after June 1, 1973, the term of service of the Director of the Federal Bureau of Investigation shall be ten years. A Director may not serve more than one ten-year term."
There are exceptions to the rule. FBI director Robert Mueller, appointed to the post by President George W. Bush just before the terrorist attacks of September 11, 2001, served 12 years in the post. President Barack Obama sought a two-year extension to Mueller's term given the nation's heightened concern about another attack.
"It wasn’t a request I made lightly, and I know Congress didn’t grant it lightly. But at a time when transitions were underway at the CIA and the Pentagon and given the threats facing our nation, we felt it was critical to have Bob’s steady hand and strong leadership at the bureau," Obama said.
And Charles Grassley said in a letter that the 2 year extension was reluctantly granted but that another extension would not be forthcoming without the consent of the whole Congress.