The United States government, through its prosecutors, has a powerful investigatory tool at its disposal, the grand jury. A grand jury is a group of citizens who hear evidence and determine whether there is probable cause that a federal crime has been committed. If they decide one has, they can issue an indictment, which will be one of milestone markers in the life of the one accused of the crime.
If you are the target of a government investigation, it is critical that you retain counsel to advise you of your rights. It's also crucial that you not discuss the case with anyone, especially anyone representing the government, without first consulting with a federal criminal defense attorney.
If you're contacted by government agents about an investigation, but are not a target, our advice is that you retain an attorney before speaking with anyone. A witness to an investigation can become a target overnight. An experienced federal criminal defense attorney may be able to ensure that you won't be charged with any crimes provided you cooperate with the government's ongoing investigation. While it may be expensive, having a federal criminal defense lawyer navigate you and advise you throughout the process will likely pay off in spades.
If you are called to testify before a grand jury, your testimony will be given in secret and you can't discuss what you were asked or what you answered to anyone, except your own attorney. Again, although you may be just a witness, it is better for you if you retain counsel to advise you throughout the proceedings.