49 CFR §107.309

1. (a)The Associate Administrator may issue a warning letter to any person whom the Associate Administrator believes to have committed a probable violation of the Federal hazardous material transportation law, the Federal Water Pollution Control Act, or any provision of this subchapter, subchapter B of this chapter, subchapter C of this chapter, or any special permit issued thereunder.
2. (b)A warning letter issued under this section includes:
   1. (1)A statement of the facts upon which the Associate Administrator bases its determination that the person has committed a probable violation;
   2. (2)A statement that the recurrence of the probable violations cited may subject the person to enforcement action; and
   3. (3)An opportunity to respond to the warning letter by submitting pertinent information or explanations concerning the probable violations cited therein.