http://www.uscourts.gov/journalistguide/district_criminal.htmlIndictment
Before the target of an investigation is arrested, prosecutors generally will take the evidence they have gathered to a grand jury. Grand juries are composed of 16 to 23 citizens. Agreement by a bare majority is required to find probable cause exists that a crime was committed.
Grand juries are formally supervised by a district judge, often the chief judge, but for all practical purposes they function day-to-day under the auspices of the U.S. Attorney's Office. Only prosecutors present evidence before a grand jury, and a finding of probable cause – necessary to issue an indictment – is a relatively low standard of proof.
The indictment lists the crimes the defendant allegedly committed and describes the facts the government believes support those allegations. It is a roadmap to what the prosecution intends to prove at trial. Grand jury indictments are returned to the district court – usually to a magistrate judge – in a sealed court hearing. Indictments generally are unsealed after a defendant is arrested.
A criminal case also can begin without an indictment. In these cases, the lead investigator swears out a criminal complaint, called an "information," setting forth the same kinds of allegations and facts that would be contained in an indictment. Absent an indictment, the prosecution must convince a judge that there is probable cause to proceed with the case. These hearings are held in open court after the defendant has been arrested in a felony case. A defendant can agree to waive indictment and proceed with the case based on the criminal complaint, or can demand that the prosecutor seek an indictment.