The disciples didn't know what to make of Jesus'
announcement of a betrayer. They began to say,
"Master, is it I?"

Peter would have questioned Him but He already
was dumbed by the foot-washing.

John was closer to Jesus so Peter nudged John
to ask whom He meant. John did and Jesus
responded, "He it is to whom I shall give a sop,
when I have dipped it." Then He gave it to Judas.