Investigators now believe the two Boston bombing suspects planned to set off explosives in New York's Times Square, according to security sources.
Police Commissioner Raymond Kelly told journalists at a briefing that the two suspects had a pressure cooker bomb and five pipe bombs they intended to set off.
He said Dzhokhar Tsarnaev told Boston investigators from his hospital bed that he and his brother had discussed going to New York to detonate their remaining explosives.
Dzhokhar travelled to New York at least once last autumn. There is a photo of him in Times Square. His older brother, Tamerlan, 26, died in a shoot out with police.
Police Commissioner Kelly said: "Dzhokhar Tsarnaev revealed he and his brother decided spontaneously on Times Square as a target and they would drive to Times Square that same night.
"They discussed this while driving around in an SUV they had hijacked after they shot and killed a MIT police officer in Cambridge.
"That plan fell apart when they realised the vehicle they hijacked was low on gas and ordered the driver to stop at a gas station when the driver used the opportunity to escape and called police.
"Up till that point the brothers had at their disposal six improvised explosive devices.
"One was a pressure cooker bomb similar to the two that exploded the marathon. The other five were pipe bombs. We knew Dzhokhar was pictured in Times Square with friends on or before April 18 2012 and he was in the city again in 2012
"We don't know if those visits were related in any way to the brother’s spontaneous decision to target Times Square."
A senior US law enforcement source told Sky News the brothers' plan to attack New York was "aspirational at best". He said it was "just talk, there were no real plans, research on, or attempts to get to New York".
The news came just hours after the mother of the two brothers allegedly responsible for the Boston Marathon bombings launched an impassioned attack on US authorities over the death of her older son.
Zubeidat Tsarnaeva said she regretted moving to the United States and claimed "America took my kids away from me".
An emotional Mrs Tsarnaeva said she was told her she could not see her 19-year-old son Dzhokhar, who is in hospital where US officials said he is being questioned by investigators.
She said: "They already told us that they are never going to show us Dzhokhar even if we come there, until he will be put into their jail we wouldn't be able to see him."
She said her sons were "nice boys" who "loved each other", and said they had been happy in America and had plenty of friends.
Mrs Tsarnaeva said the family had moved to the US because she thought "America was going to protect us".
She said Dzhokhar's lawyers had said that investigators had not started to question her son because he was not well enough.
She said she had been told he had a "really bad wound to his right neck" which meant he could not eat and was being fed by a tube.
Reports in the US have claimed the teenager suffered a self-inflicted throat injury during a shootout and subsequent stand-off with the police.
According to US officials, he said his brother Tamerlan, who died in a gunfight with police, recruited him to take part in the attacks only recently.
However, both Mrs Tsarnaeva and her husband, Anzor Tsarnaev, said there was no way their sons were responsible for the attack which killed three people, including an eight-year-old boy, and injured more than 180 others.
She said her sons were victims of a conspiracy and had been framed. She claimed she had seen a video of Tamerlan being arrested and was later shown pictures of him alive.
Mrs Tsarnaeva said she had spoken to her son after the bombings and before he was killed in the police shootout during which he told her "Don't worry mamma" and tried to reassure her he was safe.
Mr Tsarnaev told reporters: "I am going to the United States. I want to say that I am going there to see my son, to bury the older one. I don't have any bad intentions. I don't plan to blow up anything."
Banging the table as he spoke, he said: "I am not angry at anyone. I want to go find out the truth."
Mrs Tsarnaeva said she was not sure whether she would accompany her husband. She was charged with shoplifting in the US last summer and is concerned she could be arrested.
They were speaking as it emerged that Tamerlan's name had been included on a database of suspected terrorists by the CIA in 2011, 18 months before the attacks.
He was investigated after Russia's FSB security service raised concerns that he had become a follower of Russian Islam.
Mrs Tsarnaeva said she did not believe that Russia had raised concerns over her son with the US authorities.
The press conference came as Russian President Vladimir Putin gave his annual question-and-answer session during which he said the Boston bombings showed the need for closer cooperation between Moscow and Washington.
He said: "We always have said that we shouldn't limit ourselves to declarations about terrorism being a common threat and engage in closer cooperation.
"Now these two criminals have proven the correctness of our thesis."