The feds have the statute of limitations to file charges which is usually 4 years from the last event. It could be that he is "cooperating" and the feds don't want to interfere with that, or it could be that they are still investigating and gathering more evidence before they indict him. You need to talk to an attorney about this. This charge carries a mandatory 10 year minimum, so having an attorney now is crucial.
There could be a lot of reasons why someone is not indicted after an arrest. Under federal law, a person can be arrested before indictment. The government must indict within thirty days or the case is dismissed without prejudice - which means the person can still be indicted. However, the Court may find excludable time which will extend the period. Excludable time is like time outs in football. The clock and calendar continue to move but the days don't count. It is dangerous to suggest that the reason for the delay is that the defendant is cooperating - that can get someone killed! While that is ONE possibility, another equally likely is that the defense attorney waived speedy indictment in an attempt to convince the prosecutor to drop the case. Once the case is indicted it is harder for the prosecutor to discontinue it.. Good luck.
THESE COMMENTS ARE NOT LEGAL ADVICE. They are provided for informational purposes only. Actual legal advice can only be provided after consultation by an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation.