Sforno on Deuteronomy 21:7/ספורנו על דברים כ״א:ז׳

1ידינו לא שפכו את הדם הזה, we have not left a stone unturned in (making public) locating the murderer in the land.

2ועינינו לא ראו, we are certain that the murderer did not commit this act where he could be seen. Had he been seen, he would have been challenged and prevented from committing the deed. At the very least, such witnesses would have come forward.