Dank's text messages would add far far far more to the investigation than a Dank interview
Potentially, yes, and Dank
can be compelled to hand over documents (e.g. records of text messages, if he has them) and
can't use the potential for self-incrimination as grounds to refuse.
If he does hand them over there are limits to whether / when those documents can be used against him in court though.
13D Self-incrimination
(1) An individual is excused from complying with a requirement to
answer a question or to give information if the answer to the
question or the information might tend to incriminate the
individual or expose the individual to a penalty.
(1A) A person is not excused from producing a document or thing as
required by a disclosure notice given to the person on the ground
that the document or thing might tend to incriminate the person or
expose the person to a penalty.