“If a dead body is found…” (Devarim 21:1)
It is a positive commandment that when a killed body is found in a field or on the way, and it is not known who killed him, that we should behead a calf in a rough valley (“Nachal Eisan”). About this the Torah says “If a dead body is found…” etc. until the end of the paragraph.
Amongst the roots of this commandment are so that a person should be aroused when he sees this tremendous deed, when there is a gathering of all of the elders and great men of the town and they take a cow which is a large animal and go out of the town with a large number of people and crowds as all want to see this act, and at the sound of the beheading all of the listeners tremble, and they start to consider the matter. It will follow that if anyone knows any information about the matter, his heart will start to become confused and he will be aroused to go before the judges and say that which he knows, and through this the evil men and the murderers will be destroyed from our midst. Apart from this knowledge there is another achievement in this great performance in that we show with great publicity and noise that the elders and men of knowledge wish to find the murderer to be able to punish him.
Amongst the laws of this commandment are that Yerushalayim does not bring this calf, and if the body is found near the border or near a town most of whose inhabitants are not Jewish, one does not bring the calf. If there are two cities nearby where there are more people in the city which is slightly more distant from the body, one follows the bigger city even though it is further away. On measures the distances from the nose of the dead body.
This commandment applies in Eretz Yisrael when it was settled, and also on the other side of the Yarden. It is incumbent on the men, and especially on the great men of the town, as it says “and the elders of that city will take…”.