U.S. officials were expected to brief the Senate on the investigation Thursday. That same day, the suspects' parents, Anzor Tsarnaev and Zubeidat Tsarnaeva, plan to fly to the U.S. from Russia, the father was quoted as telling the Russian state news agency RIA Novosti. The family has said it wants to take Tamerlan's body back to Russia.
It is unclear whether the issue of their younger son's constitutional rights will matter since the FBI say he confessed to a witness. U.S. officials also said Wednesday that physical evidence, including a 9 mm handgun and pieces of a remote-control device commonly used in toys, was recovered from the bombing scene.
But the debate over whether suspected terrorists should be read their Miranda rights has become a major sticking point in the debate over how best to fight terrorism. Many Republicans, in particular, believe Miranda warnings are designed to build court cases, and only hinder intelligence gathering.
Christina DiIorio Sterling, a spokeswoman for U.S. Attorney Carmen Ortiz, said in an email late Wednesday, "This remains an ongoing investigation and we don't have any further comment."
Investigators have said the brothers appeared to have been radicalized through jihadist materials on the Internet and have found no evidence tying them to a terrorist group.
U.S. investigators traveled to the predominantly Muslim province of Dagestan in Russia and were in contact with the brothers' parents, hoping to gain more information.
They are looking into whether Tamerlan, who spent six months in Russia's turbulent Caucasus region in 2012, was influenced by the religious extremists who have waged an insurgency against Russian forces in the area for years. The brothers have roots in Dagestan and neighboring Chechnya but had lived in the U.S. for about a decade.