It is well known who gave the orders to shoot. Was the order to shoot justified? Were there worning shots fired first? These questions are easy to answer, as was done by the Or Commission. What other "physical evidence" are the investigators looking for? Do they want to identify each rifle that fired the shot? What for? Do they suspect some hidden non-police shooters hiding on some grassy knolls that fired to kill? Do they suspect that some of the dead actually died of heart attacks? (The cause of death was determined legally in any case.) Indict the commanders if they gave the orders to shoot without warning, which they did according to the Or commission. No need to indict lower level cops.
Report: China silences human rights criticism through intimidation at UN (Reuters)