Supervisory Special Agent was a rank within the Federal Bureau of Investigation.

In 1993, Marty Neil was promoted to the rank of Supervisory Special Agent and was recruited into Foreign Counter Intelligence at the New York City Bureau, after he had a successful hunch regarding the World Trade Center bombing. (TXF: "Squeeze")

Nancy Spiller is established as holding this rank in the script of "Ghost in the Machine", although she can also be heard referring to herself (in dialogue that is not included in the episode's final version) as a Special Agent. Her rank is not provided in the episode itself.
Ranks and Titles Within the FBI
Agent           Chief           Director
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