NEW YORK (NBC) -- The surviving Boston Marathon bombing suspect has told investigators that he and his brother discussed detonating the rest of their explosives in Times Square, senior law enforcement officials told NBC News on Thursday.
The surviving suspect, Dzhokhar Tsarnaev, initially told investigators that they planned to go to New York to party after the Boston attack, the officials said. The New York police commissioner also gave this account Wednesday.
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Under subsequent questioning, the officials said, Tsarnaev said that the brothers had discussed a follow-up attack on Times Square.
The officials cautioned that the idea was undeveloped. One senior official described the plan as "aspirational at most."
The discussion appeared to be spontaneous and happened last Thursday, after the brothers carjacked an SUV and before a shootout with suburban Boston police, New York Police Commissioner Ray Kelly said. At that time, they had one pressure-cooker bomb and five bombs, he said.
New York Mayor Michael Bloomberg told reporters that city officials were informed by the FBI of the turn in the investigation on Wednesday night. He said that the attackers would have encountered a heavy police presence in Times Square but said there was no guarantee that they would have been thwarted.
"We don't know if we would have been able to stop the terrorists if they had arrived here from Boston," he said. "We're just thankful that we didn't have to find out the answer."
Kelly said there were two photos of the younger Tsarnaev brother in New York twice last year - once on or before April 18, and a second time in November.
"We don't know if those visits were related in any way to what he described as the spontaneous decision to target Times Square," Kelly said.
Tsarnaev, wounded in the shootout with police before he was captured Friday, is in fair condition at a Boston hospital. His older brother and the second suspect, Tamerlan, was killed after the shootout.
Three people were killed and more than 200 wounded when two bombs went off near the marathon finish line April 15.
The subsequent questioning happened before Tsarnaev was read his rights, the officials said. The U.S. government had invoked an exception to the requirement that suspects be read what are known as their Miranda rights, including the right to remain silent and the right to a lawyer, before questioning. The exception is applied in cases of public danger.
A man who authorities say was carjacked by the Tsarnaev brothers before the police shootout told investigators that he heard one of the men say "Manhattan" before he escaped. Authorities have cautioned that the suspects were speaking in a Russian dialect and that it may have been a language mixup.
Times Square was the site of an attempted car bomb on May 1, 2010, that never went off. Faisal Shahzad, a Pakistani-born naturalized American citizen, was arrested as he tried to leave the country and was later sentenced to life in prison in the attempted attack.
Kelly told reporters Wednesday that his department had pictures of Dzhokhar Tsarnaev in Times Square with four other people in November 2012. But the nature of that photo was not clear.
Earlier Thursday, the Tsarnaev brothers' mother, Zubeidat Tsarnaeva, insisted from Russia that her sons were innocent.
"America took my kids away from me," she said. "I'm sure my kids were not involved in anything."
U.S. investigators want to know more about a six-month trip to Russia that Tamerlan Tsarnaev took last year. In 2011, the Russian government asked U.S. authorities twice for more information about the elder Tsarnaev, who is of Chechen descent but had a U.S. green card, because they suspected ties to extremism.
The FBI found no indications of terrorism and asked Russia to elaborate on its concerns, but Russia never answered, U.S. officials have said.