"Indictment" is a scary word for some people, but actually it's just an official way of accusing someone of a felony crime. Indicted means the case has gone before a grand jury of up to 23 people, and those people believe that the suspect probably has committed the crime he's accused of and should be brought to trial. A sealed indictment is the same thing done in secret, so the offender doesn't know he's about to be arrested.
Sealing an indictment is just a fancy way of saying that criminal charges have not been made public yet. No one knows who the offender is or what crime the charges will bring.
When someone is suspected of committing a federal crime, prosecutors will go before a grand jury and ask them to bring criminal charges against the suspect. They do this by presenting witnesses and key evidence to prove their case. It's the grand jury's job is to decide whether there's enough evidence to justify having a full criminal trial. If there is probable cause that the suspect committed the crime, the grand jury will issue an indictment. This is not a determination of guilt, but a legal statement that a crime was committed, the accused probably did it, and there should be a full legal trial.
Sealed Indictment Explained
When an indictment is issued on someone, it means he will definitely be arrested and charged with the crime. A sealed indictment is simply an indictment that is kept secret from the public. At this point, no one can disclose the existence of the indictment so no one knows who is under investigation for a crime and what offense he allegedly has committed. At some appropriate moment which prosecutors decide, the indictment is unsealed and a warrant is issued for the suspect's arrest. So, while an indictment may start out sealed, it will become unsealed before the defendant goes to trial.
Sealed Indictments Reduce Flight Risk
Sometimes, the police will arrest a suspect before presenting the evidence to a grand jury. There's no point issuing a sealed indictment in this scenario since the suspect is already in police custody or released on bail. Other times, prosecutors will appear before a grand jury before the individual even knows he's under investigation. Now, a sealed indictment will stop the suspect from discovering that he's being investigated and fleeing the jurisdiction. The grand jury may also return a sealed indictment to protect the identities of witnesses or to buy time so the police can investigate people complicit in crimes.
Only for Felony Crimes
If someone is indicted by a grand jury, then the charges typically will include at least one felony crime. The Fifth Amendment to the U.S. Constitution prevents "capital" or "infamous" crimes being charged without presentment to a grand jury – that's why many states use them to charge felonies. Other states use a comparable process called a preliminary hearing, in which a lower court judge reviews the evidence to determine whether there's enough evidence that the suspect has committed a felony. If there's probable cause, she will "indict" or order the case be sent to the appropriate court for trial. For offenses like sex crimes and drugs conspiracy, there's a fair chance the indictment will be sealed at first instance.