When the younger brother, Dzhokhar, was forced to go inside the Shell Food Mart to pay, older brother Tamerlan put his gun in the door pocket to fiddle with a navigation device — letting his guard down briefly after a night on the run. Danny then did what he had been rehearsing in his head. In a flash, he unbuckled his seat belt, opened the door, stepped through, slammed it behind, and sprinted off at an angle that would be a hard shot for any marksman.

“F—!” he heard Tamerlan say, feeling the rush of a near-miss grab at his back, but the man did not follow. Danny reached the haven of a Mobil station across the street, seeking cover in the supply room, shouting for the clerk to call 911…

Danny, trained as an engineer, made scrupulous mental notes of street signs and passing details, even as he abided the older Tsarnaev’s command not to study his face.

“Don’t look at me!” Tamerlan shouted at one point. “Do you remember my face?”

“No, no, I don’t remember anything,” he said.

Tamerlan laughed. “It’s like white guys, they look at black guys and think all black guys look the same,” he said. “And maybe you think all white guys look the same.”

“Exactly,” Danny said, though he thought nothing of the sort.