WASHINGTON -- 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.
Investigators believe the brothers were likely planning other attacks based on the cache of weapons uncovered, the city's police commissioner said Sunday.
Commissioner Ed Davis told CBS' "Face the Nation" that authorities found an arsenal of homemade explosives after a gun battle between police and the suspects in the Boston suburb of Watertown early Friday.
"We have reason to believe, based upon the evidence that was found at that scene -- the explosions, the explosive ordnance that was unexploded and the firepower that they had -- that they were going to attack other individuals," Davis said. "That's my belief at this point.”
"There were over 250 rounds of extended ammunition that was found at the scene. This was a five- to 10-minute gun battle that occurred there, punctuated by loud explosions," Davis said, adding that the explosive devices were homemade.
The scene was loaded with unexploded bombs, and authorities had to alert arriving officers to them and clear the scene, Davis said. One improvised explosive device was found in the Mercedes the brothers are accused of carjacking, he said.
"This was as dangerous as it gets in urban policing," Davis said.
He said on "Fox News Sunday" that authorities cannot be positive there aren't more explosives that haven't been found. But the people of Boston are safe, he said.
The federal Bureau of Alcohol, Tobacco, Firearms and Explosives is tracing the weapons to try to determine how they were obtained by the suspects.
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.