An FAA Letter of Investigation (LOI) is a formal notification sent to a pilot, mechanic, or aviation entity stating that the Federal Aviation Administration is looking into a potential violation of Federal Aviation Regulations. Receiving an LOI can be incredibly stressful for a professional. It essentially says, "We have information suggesting you did something wrong, and we are giving you a chance to explain your side." It is the first step in an enforcement action. The recipient usually has 10 days to respond. Legal experts often advise pilots to be very careful with their response, as anything said can be used as evidence. It doesn't automatically mean you will lose your license—it could lead to a simple warning letter, a fine, or a suspension, depending on the severity of the incident and how the pilot handles the investigation.