(a) Investigations may be initiated in any case in which it appears that there are reasonable grounds to believe that the holder of a credential or endorsement issued by the Coast Guard may have:

(1) Committed an act of incompetency, misconduct, or negligence while acting under the authority of a credential or endorsement;

(2) Violated or failed to comply with subtitle II of title 46, U.S.C., a regulation prescribed under this subtitle, or any other law or regulations intended to promote marine safety or to protect the navigable waters, while acting under the authority of a credential or endorsement;

(3) Been convicted of a dangerous drug law violation, or has been a user of, or addicted to the use of, a dangerous drug, so as to be subject to the provisions of 46 U.S.C. 7704.

(b) In order to promote full disclosure and facilitate determinations as to the cause of marine casualties, no admission made by a person during an investigation under this part or part 4 of this title may be used against that person in a proceeding under this part, except for impeachment.