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