Surveillance video from the Boston Marathon attack shows one suspect dropping his backpack and calmly walking away from it before the bomb inside exploded, Massachusetts Gov. Deval Patrick said Sunday.
The video clearly puts 19-year-old Dzhokhar Tsarnaev at the scene of the attack, Patrick said on NBC's "Meet the Press."
"It does seem to be pretty clear that this suspect took the backpack off, put it down, did not react when the first explosion went off and then moved away from the backpack in time for the second explosion," Patrick said. "It's pretty clear about his involvement and pretty chilling, frankly."
Patrick said he hadn't viewed the videotape but had been briefed by law enforcement officials about it.
Investigators have determined the bombs were fashioned from pressure cookers packed with explosives, nails and ball bearings and hidden in black backpacks. Three people were killed and more than 180 injured when the two bombs exploded Monday about four hours into the race.
Tsarnaev was captured Friday after being pulled bloody and wounded from a tarp-covered boat in a suburban Boston backyard. He is being guarded by armed officers while he recovers at a Boston hospital. He is in serious condition and hasn't been able to communicate with investigators.
His 26-year-old brother and alleged accomplice, Tamerlan, died earlier Friday after a gunbattle with police.
The brothers are also suspected of killing an MIT police officer Thursday and severely injuring a transit officer.
Patrick said Sunday on CBS' "Face the Nation" that law enforcement officials believe the immediate threat ended when police killed Tamerlan Tsarnaev and captured Dzhokhar.
The governor said he has no idea why someone would deliberately harm "innocent men, women and children in the way that these two fellows did."
On Saturday, Patrick appeared on the field at Fenway Park with dozens of local and state police before the Boston Red Sox's first home game since the bombings.