From the car, Linskey called his boss, who immediately figured out what was happening. As soon as Davis heard the word “amputations,” he said, he began to treat the episode as terrorism.

“A power station explosion wouldn’t cause that type of injury. That’s classic low-placement IED explosion,” Davis said.

Within five minutes, Davis said, as he tore down Turtle Pond Parkway back toward the heart of the city, he called Richard DesLauriers, agent in charge of the Boston office of the FBI.

“Rick, look, I don’t know what I’ve got,” Davis recalls telling the FBI chief, “but I have multiple explosions . . . I need to roll whatever SWAT teams you have available to Copley Square.”…

The street was bloody chaos, a battleground. Bodies and body parts were strewn on the pavement, and victims covered with towels. We ran out of ambulances, Linskey remembers someone telling him. The street was littered with shrapnel, nails and ball bearings, and belongings dropped by fleeing spectators: cellphones and backpacks; duffel bags and handmade signs with runners’ names.

Arriving on Boylston, Davis was struck by the contrast, the dramatic transformation: “No one but uniforms at this place where I had just been with 100,000 people.”