I think you are wrong about that....I think he had already been FBI Director and then KEPT him longer than the 10 year limit.
He was kept on long than the normal term, but that decision happened ten years after September 11th.
From Wikipedia:
"In 1998, Mueller was named U.S. Attorney for the Northern District of California and held that position until 2001.
"Federal Bureau of Investigation
"The Senate unanimously confirmed Mueller as FBI director on August 2, 2001, voting 98–0 in favor of his appointment. He had previously served as acting deputy attorney general of the United States Department of Justice (DOJ) for several months before officially becoming the FBI director on September 4, 2001, just one week before the September 11 attacks on the World Trade Center and the Pentagon.
...
"In May 2011, President Barack Obama asked Mueller to continue at the helm of the FBI for two additional years beyond his normal 10-year term, which would have expired on September 4, 2011. The Senate approved this request 100–0 on July 27, 2011."