EGLAH ARUFAH
דברים פרק כא, ט יִ כ
ךֶּ בְׁרִ
קִמ יִקָּ נַה םָּ דַה רֵּעַבְׁ ת הָּ תַאְׁו-ה יֵּניֵּעְׁ ב רָּ שָּ
יַה הֶּ שֲעַת'
ירושלמי מסכת סוטה דף מג/א פרק ט הלכה ו נערפה העגלה ואח"כ נמצא ההורג יהרג
מה כתיב "ונכפר להם הדם" ושתק אלא אפילו כן ואחר כך אמר "ואתה תבער הדם הנקי מקרבך"
One might say that once the blood of the Eglah has brought atonement, then, no further mitzvah exists. Therefore, the verse alludes to the concept that if the murderer is found, he would still be put to death.
T.T. The Rambam explains the purpose of the ceremony with the Eglah is to publicize the unsolved death, which might lead to the revealing of the murderer. Similar to the words of the Targum Yonasan ben Uziel, that the noise of the valley (echo) would not stop until it reaches all places, like the worm that goes into all the crack and crevices that are hidden from a person.
