Just weeks earlier, an agent and Boston officers had shot and killed an ISIS sympathizer on the same streets, right before he boarded a bus armed with a military-style knife and plans to attack cops and behead a woman.

Now, an undercover operative set up a weapons buy on the streets with a young Massachusetts man, the son of a respected cop but now someone who had been radicalized by the Islamic State. Agents believed he was plotting to shoot or blow up students at an out-of-state campus with bombs laced with Styrofoam, which sticks to skin and worsens the burns.

The buy went down, the young man was arrested and a search of his apartment quickly confirmed agents' fears.