Once you have been arrested on a Federal Complaint and Warrant, the government must hold a preliminary exam with 14-21 days unless you consent, and good cause is shown. Otherwise, the rules require that you be released. However, you can only be held on a complaint. You cannot be prosecuted further on a complaint and warrant. To prosecute you further, the government must either file an information or obtain an indictment.
To better understand this, it is helpful to consider that when the government believes that you have committed a felony over which the federal courts have jurisdiction, the prosecution for this crime may be initiated by the government in one of three ways. The most common of the three is the criminal indictment. However, in certain circumstances, the government may determine that there is a need to forgo the grand jury and instead will prepare and file a complaint. This procedure is governed by Rules 3-5.1 of the Federal Rules of Criminal Procedure.
A compliant, and the necessary probable cause to support it, may be based in whole or in part on hearsay. According to Rule 3 of the Federal Rules of Criminal Procedure, a complaint must contain the essential facts of the crime alleged, and must be presented to a magistrate judge, under oath. Rule 4 of the Federal Rules of Criminal Procedure provides that, in reviewing the warrant and deciding whether to issue an arrest warrant, the magistrate judge must determine whether the complaint establishes probable cause to believe 2 things; first that the crime alleged has been committed and that second that the defendant committed it.