Starfleet Security was a branch of Starfleet responsible for safeguarding the Federation from outside threats, and for conducting criminal investigations.
Harris was a member of Starfleet Security until 2149, at which point he became a member of the covert operations group known as Section 31.
In 2154, Commander Collins was an officer in Starfleet Security. When Doctor Phlox was abducted in San Francisco as part of a Klingon plot, she was in charge
of the investigation.
In the 2350s, a secret group at Starfleet Security engaged in an illegal operation to develop a Starfleet phasing cloaking device, in direct violation of the Treaty of Algeron. The phasing cloak was tested aboard the USS Pegasus in 2358, under the command of Captain Erik Pressman, but ended in disaster when it appeared the Pegasus was destroyed. The entire affair was subsequently covered up by Starfleet Intelligence. Twelve years later, the Chief of Starfleet Security at the time, Admiral Raner, oversaw an operation to retrieve the experimental cloaking device.
In 2367, Vice Admiral Thomas Henry, who was attached to Starfleet Security, boarded the USS Enterprise NCC-1701-D to observe a hearing overseen by former Starfleet admiral Norah Satie, concerning an attempt to uncover a supposed conspiracy at the heart of Starfleet.
In 2371, due to skepticism related to Constable Odo’s abilities as security chief, Starfleet Security assigned Lieutenant Commander Michael Eddington to Deep Space 9, to oversee security issues pertaining to Starfleet matters.
Starfleet Security often sent Odo criminal activity reports.
A Starfleet Security investigation determined that Ambassador Krajensky had been kidnapped or killed by the Dominion on his way to Risa.
In 2372, Admiral Leyton made Benjamin Sisko acting head of Starfleet Security, in face of the apparent Changeling infiltration of Starfleet. In this capacity, Sisko discovered that Leyton was using the recent bombing of the Antwerp Conference as an excuse to attempt to seize power by the military, by instilling paranoia about the threat posed by the Dominion. Sisko subsequently uncovered Leyton’s conspiracy, forcing him to resign, and returned to Deep Space Nine.
After Sisko was attacked by a member of the Cult of the Pah-wraiths in early 2375, he gave a statement to Starfleet Security.