Other than Section 22 Police Record which pertains to charges against you and Section 28 which asks for involvement in any public record non-criminal civil court proceedings not listed elsewhere, there is not a question that requires this information. An FBI investigation is criminal and not a public record, so in my mind you were not required to disclose it. FBI name checks come back with anyone involved in one of their cases. As long as you were not the one being investigated and had no part in any wrong-doing and you conveyed that during your interview, it should not be an issue that would result in a denial.