We understand that you are seeking prayers and answers regarding a past event that occurred on Sunday, May 28, 2017, which you describe as evil and against God's creation. We commend you for coming in Jesus' name, as it is only through Him that we have access to God and salvation. As it is written in Acts 4:12 WEB: "There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved!"
We must address the fact that you mentioned "retroactive prayers." This concept is not biblical. Prayer is a present communion with God, and while we can pray for healing, comfort, and justice in regards to past events, we cannot go back in time. However, God is outside of time, and He is aware of all things past, present, and future. We can trust that God was present in that situation and that His will was done.
Regarding the evil committed by the person hiding in anonymity, we must not take vengeance or hatred but instead pray for this individual. As it is written in Matthew 5:44 WEB: "But I tell you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who mistreat you and persecute you,"
Let us pray together:
"Father, we come before You in Jesus Christ's mighty name, seeking Your will in this situation. Lord, we know that You were present on that Sunday, May 28, 2017, and that You are aware of the evil committed. We pray for the individual who committed this act, that they would come to know You, Lord, and turn from their wicked ways. We also pray for those affected by this event, that they would find healing, comfort, and justice in You, Lord. We ask for Your will to be done in this situation, and we trust that You are working all things together for good for those who love You and are called according to Your purpose. In Jesus' name, we pray. Amen."
Continue to trust in the Lord, knowing that He is faithful and just. Keep your focus on Him pray for guidance and wisdom in this situation. Psalm 37:5-6 WEB: "Commit your way to Yahweh. Trust also in him, and he will do this: he will make your righteousness go forth as the light, and your justice as the noon day sun."