The last time Congress arrested and detained a witness was in 1935. Since then, it has instead referred cases to the United States Department of Justice. . .
. . . Following a contempt citation, the person cited is arrested by the Sergeant-at-Arms for the House or Senate, brought to the floor of the chamber, held to answer charges by the presiding officer, and then subjected to punishment as the chamber may dictate (usually imprisonment for punishment reasons, imprisonment for coercive effect, or release from the contempt citation).
Sauce: https://en.wikipedia.org/wiki/Contempt_of_Congress