I hadn't noticed

“If a slain person is found lying in the open country in the land which the Lord your God gives you to possess, and it is not known who has struck him ... All the elders of that city which is nearest to the slain man shall wash their hands over the heifer whose neck was broken in the valley; and they shall answer and say, ‘Our hands did not shed this blood, nor did our eyes see it.

And When Pilate saw that he was accomplishing nothing, but rather that a riot was starting, he took water and washed his hands in front of the crowd, saying, “I am innocent of this Man’s blood; see to that yourselves.”

I'd thought it was a Roman thing, but apparently he knew a bit of the local custom.