49 CFR 831.2: Responsibility of the NTSB.

The NTSB is required to investigate—

(a) Aviation accidents as described in subpart B of this part;

(b) Highway accidents as described in subpart C of this part;

(c) Railroad, pipeline, and hazardous materials accidents as described in subpart D of this part; and

(d) Any accident that occurs in connection with the transportation of people or property that, in the judgment of the NTSB, is catastrophic, involves problems of a recurring nature or would otherwise carry out the intent of its authorizing statutes. This authority includes selected events involving the transportation of hazardous materials, including their release.