To apply to become an FBI agent, one must be between the ages of 23 and 37, unless one is a preference-eligible veteran, in which case one may apply after age 37. The applicant must also hold U.S. citizenship, be of high moral character, have a clean record, and hold at least a four-year bachelor's degree. At least three years of professional work experience prior to application is also required. All FBI employees require a Top Secret (TS) security clearance, and in many instances, employees need a TS/SCI (Top Secret/Sensitive Compartmented Information) clearance. To obtain a security clearance, all potential FBI personnel must pass a series of Single Scope Background Investigations (SSBI), which are conducted by the Office of Personnel Management. Special agent candidates also have to pass a Physical Fitness Test (PFT), which includes a 300-meter run, one-minute sit-ups, maximum push-ups, and a 1.5-mile (2.4 km) run. Personnel must pass a polygraph test with questions including possible drug use. Applicants who fail polygraphs may not gain employment with the FBI. Up until 1975, the FBI had a minimum height requirement of 5 feet 7 inches (170 cm).