My understanding is that "criminal complaint" and "indictment" are roughly interchangeable (one or the other, depending on the max penalty); and that the "preliminary hearing" (which is always post-arraignment, usually w/in weeks) is an entirely different activity. The functions of the preliminary hearing are to generally review the sufficiency of the prosecution's evidence, and to set parameters for discovery.
The timeline (order) is not fixed as in "arrest then indict" vs. "indict then arrest," so a months long investigation while the suspect is free doesn't pose the "why are you holding me, and how do I defend myself" issue at all. The intention is often that the suspect be completely unaware of being under suspicion and investigation. That fact pattern is not indicated in this case.
Federal Rules of Criminal Procedure
United States Attorneys' Manual - Title 9 (Criminal)
The juvenile statutes provide for release of a juvenile pending trial to his parents, guardian, custodian, or other responsible individual unless the magistrate determines, after a hearing at which the juvenile is represented by counsel, that detention is required to secure his timely appearance before the appropriate court or to insure his safety or that of others. 18 U.S.C. § 5034. ...The juvenile statutes have their own speedy trial provision. Juveniles who are held in custody pending trial must ordinarily have their proceedings commence within 30 days. 18 U.S.C. § 5036.
If an alleged delinquent who is in detention pending trial is not brought to trial within thirty days from the date upon which such detention was begun, the information shall be dismissed on motion of the alleged delinquent or at the direction of the court, unless the Attorney General shows that additional delay was caused by the juvenile or his counsel, or consented to by the juvenile and his counsel, or would be in the interest of justice in the particular case. Delays attributable solely to court calendar congestion may not be considered in the interest of justice. Except in extraordinary circumstances, an information dismissed under this section may not be reinstituted.
A juvenile alleged to be delinquent may be detained only in a juvenile facility or such other suitable place as the Attorney General may designate. Whenever possible, detention shall be in a foster home or community based facility located in or near his home community. The Attorney General shall not cause any juvenile alleged to be delinquent to be detained or confined in any institution in which the juvenile has regular contact with adult persons convicted of a crime or awaiting trial on criminal charges. Insofar as possible, alleged delinquents shall be kept separate from adjudicated delinquents.
I'm thinking there must be a parallel set of laws to cover this case, because it sure doesn't appear to be tracking the ordinary criminal statutes.
My first reaction was to correlate "juvenile delinquent" with petty activity, but the law defines it as the violation of a law of the United States committed by a person prior to his eighteenth birthday which would have been a crime if committed by an adult. This includes everything up to and including cold murder.