From cnn.com: Aided by a federal grand jury in White Plains, New York, the investigation has detained 80 people for questioning and has four people under arrest as material witnesses.

So they arrest a material witness, which implies wrongdoing, as if the witness were also invovled in the crime. Or maybe they are "arrested" for safekeeping -- i.e. sequestered so somebody like the mafia or a terrorist can't "grease" them.