TT #333 · Parashas Shoftim · Seventh Aliyah · Passuk 21:01
If a slain person is found in the land that the Eternal your God is giving you to possess, lying in the field, and it is not known who struck him.
Torah Temimah points to Sotah 45b / Sifrei.
TT lemma: לֹא נוֹדַע מִי הִכָּהוּ.
English: ‘It is not known who struck him’ – but if it is known, even if the killer is at the end of the world, they do not break the neck.
Derash on the condition of unknown killer (Bavli / Sifrei) — once the identity is known the eglah is not performed.
If the identity of the killer becomes known, even if he is far away, the eglah arufah is not carried out.