18 USC 3146, makes it a Federal crime or offense for anyone who has been released on bail to thereafter willfully fail to appear when required to do so.
A person can be found guilty of that offense only if all of the following facts are proved:
First: That the person had been admitted to bail pursuant to an order of a Judge or Magistrate of a Court with proper juridiction;
Second: That the person thereafter failed to appear before a Judge or Magistrate of a Court with proper juridiction as required; and
Third: That the person did so knowingly and willfully.