At first glance, Dzhokhar Tsarnaev appeared to be just another teenager standing along the route of the Boston Marathon moments before the carnage of last week's bombings.
Tsarnaev, 19, was seen with a white baseball cap on backwards, a backpack slung casually over his shoulder, and endlessly fiddling with his cellphone.
But the University of Massachusetts student's ice-cold reaction to the first of the two bomb blasts -- caught in detail by surveillance cameras -- left FBI agents convinced of his involvement in the terror attacks.
While people around him react with shock and panic at the first blast, Tsarnaev ignores the mayhem before calmly walking away in the opposite direction, having planted the second rucksack bomb -- a pressure cooker packed with low-grade explosive, nails and ball-bearings -- by the curb.
Tsarnaev's chilling detachment was recounted in an affidavit by FBI special agent Daniel Genck released on Monday after the teenager was charged with federal offenses over the bombings that left three people dead and 200 injured.
Genck said that a study of surveillance footage showed Tsarnaev -- identified as "Bomber Two" -- appearing to make a cell phone call shortly before the first explosion.
"A few seconds after he finishes the call, the large crowd of people around him can be seen reacting to the first explosion," Genck states.
"Virtually every head turns to the east (towards the finish line) and stares in that direction in apparent bewilderment and alarm.
"Bomber Two, virtually alone among the individuals in front of the restaurant, appears calm. He glances to the east and then calmly but rapidly begins moving to the west, away from the direction of the finish line.
"He walks away without his knapsack, having left it on the ground where he had been standing."
Approximately 10 seconds later, Genck states, an explosion occurs in exactly the spot where Tsarnaev had left his backpack.
Tsarnaev could face the death penalty if convicted over the attacks he is alleged to have carried out with his elder brother Tamerlan Tsarnaev, who was killed during a dramatic confrontation with police last week.
The affidavit also painted a picture of the chaotic scenes that unfolded late Thursday when the two brothers scrambled to elude law enforcement as the net closed around them.
A few hours after the FBI had taken the step of publishing video and photographic images of the two men, one of them carjacked a man at gunpoint in Cambridge, Massachusetts.
The motorist was sitting in his car when one of the Tsarnaev brothers approached, and tapped on a window before forcing his way into the vehicle.
"The man pointed a firearm at the victim and stated 'Did you hear about the Boston explosion?' and 'I did that.'"," according to Genck's affidavit.
"The man removed the magazine from his gun and showed the victim that it had a bullet in it and then re-inserted the magazine. The man then stated 'I am serious.'"
After the carjack victim managed to escape at a gas station, the Tsarnaevs fled in a high-pursuit with police in which they threw at least two home-made bombs out of the car before exchanging gunfire.
Tamerlan Tsarnaev was killed in the exchange; Dzhokhar Tsarnaev was arrested on Friday after being discovered hiding in a boat parked in the driveway of a Boston home.