High-definition video cameras with 30x magnification keep watch over the Boston Marathon finish line, where two bombs detonated in 2013, killing three people and injuring hundreds. Jesse Costa/WBUR hide caption

itoggle caption Jesse Costa/WBUR

Micaela Torres and 2-year-old Jakai Johnson swing underneath a surveillance camera at Miwok Park in Elk Grove, Calif. The city's police department collects more than 100 video feeds from across the city. Steve Henn/NPR hide caption

itoggle caption Steve Henn/NPR