Federal employees are in a vulnerable position for an investigation by an Office of Inspector General (or OIG). Basically, an OIG investigation can run in two different directions. Each has it's own dangers that a federal employee who hears from an OIG Agent needs to be aware of.
If an OIG Agent is investigating a criminal violation of law, then the federal employee has the risk of being prosecuted. If the OIG Agent thinks he or she can prove that the federal employee committed a crime, and the OIG Agent can convince an Assistant United States Attorney to bring a case, then the federal government is bringing its resources to bear to convict the federal employee of a crime. Often, this means that the government wants a felony conviction, and it can quickly mean that prison time is a real risk.
If, however, the Assistant United States Attorney decides that a criminal prosecution is not warranted, either because there isn't enough evidence of a crime, or because what happened isn't serious enough to warrant a prosecution, or because what the OIG Agent is investigating isn't a violation of a criminal law, then the federal employee is still not in a good position, because he or she can lose his or her job. If criminal charges aren't an option, the OIG Agent can require that a federal employee give an interview. If the employee doesn't give the interview, then that can be a basis for a disciplinary action.
Federal employees face unique risks. They're conduct has its own law enforcement offices that are set up to investigate - aside from government contractors, OIG Agents spend a lot of time looking at federal employees.
For a federal employee who is under scrutiny by an OIG Agent, it is important to know what is happening, and what needs to be done to protect your job, and, possibly, your freedom.