Should I report the murder I witnessed in my childhood, knowing that reporting it now might lead to the imprisonment of the witnesses involved with me, especially since the killer has emigrated and remains unknown to justice?

A Muslim is obligated to bear witness for Allah Almighty if requested to do so, provided they are certain and harbor no doubt, and there is no one else to bear it, and they do not fear for their life, property, honor, or family. This is based on the Almighty's saying: "And bring to witness two just persons from among you and establish the testimony for Allah" and His saying: "And do not conceal testimony, for whoever conceals it - his heart is indeed sinful." The obligation to bear witness is lifted if one is not certain, or fears for themselves, or if there is someone else sufficient to bear the testimony.