The information came to law enforcement officials from the interrogation of 19-year old DzhokharTsarnaev, who had at first said the plan was to go to New York to "party" but then changed his story in a second interview when he admitted they planned to explode their bombs.
Bloomberg and NYPD Commissioner Ray Kelly said they were informed by investigators on Wednesday night.
Kelly says the two suspects had a pressure cooker bomb and five pipe bombs they wanted to set off.
"We don't know if we would have been able to stop the terrorists had they arrived here from Boston," Mayor Michael Bloomberg said. "We're just thankful that we didn't have to find out that answer."
They 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. They said they decided it spontaneously.
"They discussed this while driving around in a Mercedes SUV that they hijacked after they shot and killed the officer at MIT," the police commissioner said. "That plan, however, fell apart when they realized that the vehicle they hijacked was low on gas and ordered the driver to stop at a nearby gas station."
The driver escaped and called police, Kelly said. That set off the gunbattle and manhunt that ended a day later with Dzhokhar captured and 26-year-old Tamerlan dead.
The suspects' father said Thursday that he is leaving Russia for the United States in the next day or two, but their mother said she was still thinking it over.
Anzor Tsarnaev has expressed a desire to go to the U.S. to find out what happened with his sons, defend his hospitalized son and, if possible, bring his older son's body back to Russia for burial.
Their mother, Zubeidat Tsarnaeva, who was charged with shoplifting in the U.S. last summer, said she has been assured by lawyers that she would not be arrested, but was still deciding whether to go.
Tsarnaev traveled to New York at least once last fall. In an ABC News exclusive, Dzhokhar was seen in a picture with friends in Times Square. NYPD Commissioner Ray Kelly says it is a significant piece of evidence for his investigators. The picture was posted in November of 2012 on a Russian social media site similar to Facebook.
Two of the friends seen in the picture are now in police custody on immigration charges.
Relatives say Tamerlan Tsarnaev may have been radicalized by an Islamic tutor in Boston named Misha, but his mother denied those accusations.
The database information adds new questions to the debate over whether authorities could have done more to prevent the attack.
Meanwhile, authorities say Dzhokhar Tsarnaev was unarmed at the time of his apprehension.
During the night, an FBI convoy carried crucial evidence into Boston. Investigators found a stash of fireworks, mostly emptied of their gunpowder, dumped in a used clothing bin in Watertown.
The large bins were loaded onto flatbed trucks and taken to an evidence processing center.
Authorities say Dzhokhar acknowledged to the FBI his role in the Boston Marathon attacks, but did so before he was advised of his constitutional right to keep quiet and seek a lawyer.
Once the 19-year-old was read his Miranda rights on Monday, he immediately stopped talking and is no longer cooperating with the investigation, according to ABC News.
After roughly 16 hours of questioning, investigators were surprised when a magistrate judge and a representative from the U.S. Attorney's office entered the hospital room and read Tsarnaev his rights.
Investigators had planned to keep questioning him. It is unclear whether any of this will matter in court, since the FBI says Tsarnaev confessed to a witness and physical evidence, including a 9 mm handgun and pieces of a remote-control device commonly used in toys, linking him to the attacks were recovered from the scene.
Pictures from the Boston Marathon bombings (The Associated Press contributed to this report)