Denial of an administrative claim under this subpart shall be in writing, and notification of denial shall be sent to the claimant, or his or her attorney or legal representative by certified or registered mail. The notification of final denial shall include a statement of the reasons for the denial and shall include a statement that, if the claimant is dissatisfied with the Department’s action, that claimant may file suit in an appropriate U.S. District Court not later than 6 months after the date of mailing of the notification.