On last year’s Patriot’s Day, a holiday celebrated every April 15 in the state of Massachusetts, two men, brothers Dzhokhar and Tamerlan Tsarnaev, detonated two bombs that killed three people and wounded dozens more who were watching the annual Boston Marathon.

The sun was already setting as students rushed to get to their next class or to head home after a tiring day of school. But that night, with candles held in their hands flickering in the darkness,...