If the case is no longer being investigated, you may be able to file a FOIA request to get part of the file. However, I highly suggest that you contact an attorney for this area.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
The answer to your question really depends on the information you are seeking and purpose for which you are seeking it. Generally a duces tecum is used to order an individual or entity to appear before the court and provide certain documents or evidence described in the subpoena. Also, it is doubtful that any judge will order anyone to appear without some sort of relevant dispute before the court. In other words a pending court case. If there is a pending civil case regarding your brother's death then there is probably an attorney involved that filed it. That attorney would have to be the one requesting the information. If there is no pending case the information you seek may be obtained through a request pursuant to the Oklahoma Open Records Act. If the agency does not respond to your request you may be able to seek an order of the court to force them to respond. Failure to respond to a request pursuant to the OORA is a misdemeanor. One more note. If the investigation into your brother's death is still ongoing, the chances of getting anything are pretty slim.