That disclosure is certain to raise questions on Capitol Hill over the Obama administration missed an opportunity to thwart the Boston attack.
Kelly, citing interrogations carried out by the task force investigating the Boston Marathon attack, said that days after the bombing, the Tsarnaev brothers "planned to travel to Manhattan to detonate their remaining explosives in Times Square,"
"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.