(CBS News) -- Boston Marathon bombings suspects Tamarlan and Dzokhar Tsarnaev had a cache of weapons that hints at plans for a followup attack and investigators now say the brothers may have headed for New York City if not for the shooting death of Tamerlan in a police standoff on Thursday night and the eventual capture of Dzokhar Friday night.
Investigators have new insight from the carjacking victim held hostage by Tamerlan and Dzokhar Thursday night. The victim speaks little to no English, according to CBS News special correspondent John Miller, but authorities pressed him to remember recognizable words from his exchange with the bombing suspects. The suspects openly boasted to the victim in English about their role in the Boston Marathon bombing and carried out the rest of their exchange in Russian.
John Miller reports that the victim said, "'the only word I recognized was Manhattan,'" a word which "tripped a lot" of alarm for authorities, who quickly halted Amtrak service from Boston to New York, searched the trains. The tip prompted the New York Police Department to "flip on its network of license plate readers at all bridges and tunnels coming into the city," Miller said. "They loaded all the license plates associated with these guys" to prevent possible entry into Manhattan.
Surviving suspect Dzhokhar Tsarnaev continued to answer questions in writing from his hospital bed Monday night. "He can say about one word at a time," Miller said.
So far, his account indicates he was "driven by his brother" and that "it was mostly done online, in terms of radicalization [and] finding instructions." Investigators say it remains unlikely there is an international terrorist organization behind the attack or the Tsarnaev's training.
The investigation also has yielded new insight about the motive for the robberies and carjacking committed by the suspects on Thursday night, a bloody night that began when they approached and shot an MIT police officer, Sean Collier, "in the head, unprovoked," Miller said.
The operating theory currently held by investigators looking to explain the assassination is "that they were short one gun, that the older brother had a gun, they wanted to get a gun for the younger brother and the fastest and most efficient way they could think of doing it was a surprise attack on a cop to take his weapon and go."
But the suspects failed in that aim, because Officer Collier had a locking holster and they were unable to remove the gun. "There was apparently an attempt to yank it," Miller said, "And they couldn't get it and left."