Authorities say Tsarnaev could get the death penalty. Christina DiIorio-Sterling, a spokeswoman for U.S. Attorney Carmen Ortiz in Boston, would not comment on whether authorities plan to add charges based on the alleged plot to attack New York.
Meanwhile in Massachusetts, the Middlesex County district attorney's office said it is building a murder case against Tsarnaev for the death of MIT police officer Sean Collier three days after the bombings.
As authorities began disclosing the suspects' plans and motives, the hospital-room questioning of Tsarnaev is generating concern about whether he should have been interrogated without first being told of his constitutional rights to stay silent and have a lawyer present — and, conversely, whether federal agents actually should have had more time with him before he was read his rights.
Tsarnaev faced 16 hours of questioning before he was advised of his Miranda rights, and investigators say he told them of his role in the two bombings near the Boston Marathon finish line. He explained that he and his brother were angry about the U.S. wars in Afghanistan and Iraq and the killing of Muslims there, according to two U.S. officials who spoke on condition of anonymity because they weren't authorized to discuss the case with reporters.
Tsarnaev also described their plan to drive to New York and set off the remaining explosives there.
In Boston, federal agents invoked an exception to the Miranda warnings that allows for questioning when public safety may be threatened. But they knew their time with Tsarnaev in the absence of a lawyer would be limited.
On Sunday, prosecutors filed a criminal complaint charging Tsarnaev with a role in the bombings. That action led directly to an improvised court hearing in the hospital the following morning at which U.S. Magistrate Judge Marianne Bowler told Tsarnaev he did not have to answer questions and could have a lawyer. He then stopped talking.