New York City Police Commissioner Ray Kelly says Boston Marathon terror suspects Tamerlan and Dzhokhar Tsarnaev were plotting to detonate more bombs in Times Square, according to the Joint Terrorist Task Force.

The surviving Boston Marathon suspect has told investigators that he and his brother decided after the attack to set off more of their arsenal — as many as six bombs — in Times Square, authorities in New York said Thursday.

The suspect, Dzhokhar Tsarnaev, first told investigators that the brothers were headed for New York to party after the marathon blasts, but he said under later questioning that they changed plans on the fly and intended a second attack there, Police Commissioner Ray Kelly said.

Mayor Michael Bloomberg told reporters that city officials were informed by the FBI of the turn in the investigation on Wednesday night. City officials were told that “the surviving attacker revealed that New York City was next on their list of targets. He and his brother had intended to drive to New York and detonate additional explosives in Times Square,” he said.

Senior law enforcement officials cautioned to NBC News that the idea was undeveloped. One senior official described the plan as “aspirational at most.”

The brothers’ decision was spontaneous and came after they carjacked an SUV and before a shootout with suburban Boston police early Friday, Kelly said. At that time, they had one bomb made with a kitchen pressure cooker and five pipe bombs, he said.

VKontakt A photograph from Dzhokhar Tsarnaev's page on VKontakt, the Russian equivalent of Facebook, appears to show him in Times Square. It is believed to be from November 2012. New York Police Commissioner Ray Kelly said authorities knew of two trips by Dzhokhar Tsarnaev to New York in 2012 but said he did not know whether those visits were related to any plot against Times Square. NBC News intentionally blurred the faces of the people with Tsarnaev.

The brothers’ plan was interrupted only because they realized the SUV was low on gas, Kelly said.

Bloomberg said that the attackers would have encountered a heavy police presence in Times Square, and that an extensive network of cameras would have seen them, but that there was no guarantee 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 Tsarnaev, who has been hospitalized since he was captured Friday, was more lucid in the second questioning and was speaking. Initially, he communicated with investigators mostly by moving his head and by writing, investigators have said.

Kelly said police knew of two visits by the younger Tsarnaev brother to New York last year — once on or before April 18, and once in November. He said he did not know whether those visits were related to any plot against Times Square.

The New York office of the FBI said there was no ongoing specific or credible threat to the city.

Tsarnaev, who was wounded in the shootout with police, is in fair condition at a Boston hospital. His older brother and the second suspect, Tamerlan Tsarnaev, was killed after the shootout. Authorities say they also shot and killed a Massachusetts Institute of Technology campus patrol officer.

Three people were killed and more than 200 wounded when two bombs went off near the marathon finish line April 15.

The subsequent questioning of Dzhokhar Tsarnaev came before he was read his rights, senior law enforcement officials told NBC News. 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.

The man 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.

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.

Ten days after the bombings, 32 people injured in the blasts remained in Boston hospitals, with one in critical condition.

NBC News In this photo obtained by NBC News, a tow truck delivers a bullet-riddled black Mercedes SUV to police headquarters in Watertown, MA, on Friday afternoon. The car was abandoned a half-mile west of the site of the gun battle, at the intersection of Spruce and Lincoln streets in Watertown.

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