Discussing the similar case of Faisal Shahzad, who attempted to bomb Times Square, Orin Kerr, a law professor who is an expert on the 4th Amendment wrote:
"Importantly, though, it would not have violated Shahzad’s constitutional rights to not read him his Miranda rights. A lot of people assume that the police are required to read a suspect his rights when he is arrested. That is, they assume that one of a person’s rights is the right to be read their rights. It often happens that way on Law & Order, but that’s not what the law actually requires. Under Chavez v. Martinez, 538 U.S. 760 (2003), it is lawful for the police to not read a suspect his Miranda rights, interrogate him, and then obtain a statement that would be inadmissible in court. Chavez holds that a person’s constitutional rights are violated only if the prosecution tries to have the statement admitted in court. See id. at 772-73. Indeed, the prosecution is even allowed to admit any physical evidence discovered as a fruit of the statement obtained in violation of Miranda — only the actual statement is excluded. See United States v. Patane, 542 U.S. 630 (2004). So while it may sound weird, it turns out that obtaining a statement outside Miranda but not admitting it in court is lawful."
Oops, link: http://www.volokh.com/2010/05/05/shahzad-and-miranda-rights/