In the weeks after Palestinian terrorists killed nearly 400 people at an Israeli music festival on Oct. 7, 2023, a growing corps of citizen-volunteers trekked to the site of the massacre to collect the personal items left behind. The group warehoused the items and organized and tagged them so survivors and the relatives of victims could retrieve them. The police saw the effort as helpful to their investigation: Once they’d studied and recorded the scene, the items could bring forward witnesses