Tradition
St. John Chrysostom
Homilies on the Gospel of John
Homily 72 › John 13:21
“When Jesus had thus said, He was troubled in spirit, and testified, and said, Verily, verily, I say unto you, that one of you shall betray Me.”
Again He brings fear on all by not mentioning (the traitor) by name.
Source: Homilies on the Gospel of John (New Advent)