49 CFR §605.32

The Administrator will allow the respondent not more than 30 days to show cause, by submission of evidence, why no violation occurred. A like period shall be allowed to the complainant, if any, during which the complainant may submit evidence to rebut the evidence offered by the respondent. The Administrator may undertake such further investigation, as deemed necessary, including the holding of an evidentiary hearing or hearings in a manner consistent with the record requirements of 49 CFR 5.81.