In most federal cases, an arrest, if it occurs at all, will only happen after a lengthy investigation and with you and your attorneys being given full notice of the impending arrest. When you are arrested, you are taken into custody. This means that you are not free to leave. Without being arrested you can be detained or held for a short period of time if a Federal agent believes you may be involved in a crime.
•    You are never required to answer any questions, except to give your name and address or show some identification if requested.
Any Federal agent can arrest you. You can be arrested whether the Federal agent is on or off duty in most cases. A probation or parole officer can also arrest you.
Typically, a Federal agent will need an arrest warrant in order to arrest you, unless he has probable cause (good reason to believe that you have committed a felony, a crime of a serious nature, usually punishable by imprisonment for more than a year) You do not need to be seen committing a felony to be arrested. However, in order for a Federal agent to arrest for a misdemeanor, they must have seen you commit such an act.