In a federal criminal case, a defendant is formally charged by way of a complaint or grand jury indictment. The charge sheet lists all the allegations a defendant is charged with. A copy is provided to the defendant when they make their initial appearance.
A complaint is a charge sheet that is based on the prosecutor's good faith belief that the defendant has committed the crime(s) alleged. Ordinarily, the complaint is supported with an affidavit for an arrest warrant by a federal agent that outlines his or her investigation and basis for the charge(s).
If an accused is brought to court on a complaint, then the defendant has the right to a preliminary hearing. A preliminary hearing is a probable cause determination hearing where the prosecutor has a relatively low burden of proving the allegations with evidence and witnesses. If the prosecutor satisfies this burden, then a defendant will be held to answer. This means the judge made an official finding that there is sufficient evidence to move forward with prosecuting the defendant.
Thereafter, the federal prosecutor will file an information. Like a complaint, an information is an official charge document, except, it is supported with sufficient evidence after a preliminary hearing is held.
An indictment is a formal charge sheet that is filed and supported by the grand jury. Prior to issuing an indictment, the prosecutor presents their evidence before the grand jury. Thereafter, the grand jury issues an indictment if the prosecutor presents sufficient probable cause that a federal crime has been committed by the defendant. The process of obtaining a federal indictment can be days, weeks, or even a year.
It is not uncommon for a defendant to initially appear in federal court with a complaint filed against them. The U.S. Magistrate Judge will then set a date for the defendant's preliminary hearing. In the interim period, the federal prosecutor will present the case before the grand jury and obtain an indictment. This normally occurs when the government believes the defendant is a risk to public safety or may flee the country.
In sum, both an information and indictment must be supported with probable cause. However, the difference between the two is who determines whether probable cause exists to warrant the charges. In other words, if a judge determines probable cause exists, then the case will proceed by way of information. If a grand jury decides sufficient probable cause exists, then the case will move forward by way of a grand jury indictment.