Updated April 21, 8:04 p.m.
Law enforcement and intelligence agencies Sunday were working critical new leads in the Boston bombing in what has become an international investigation stretching to Russia and neighboring republics. The lead suspect in the case, 19-year-old Dzhokhar Tsarnaev, remained in serious but stable condition in a Boston hospital with wounds sustained before police took him into custody late Friday.
Officials said Tsarnaev was recovering from a throat wound and unable to speak with investigators from his hospital room but was alert Sunday night and able to answer written questions, NBC News correspondent Pete Williams reported.
Federal authorities were preparing charges against Tsarnaev that would likely include committing a terrorist act with a weapon of mass destruction. Two bombs placed near the finish line of the Boston marathon killed three people and injured more than 170 last Monday.
Tzarnaev's brother and suspected accomplice, Tamerlan Tsarnaev, died in a shootout with police in the Boston suburb of Watertown early Friday. Authorities are focusing the investigation on trips that Tamerlan, who was 26, took in 2012 to Dagestan, a predominantly Muslim region of Russia in the Caucus Mountains. Law enforcement officials told NBC News that as early as 2011, Russian authorities had raised concerned about the older brother and requested that the FBI monitor him in connection with suspected terrorist activities.
Success in the fast-moving investigation was likely now to depend heavily on cooperation between Russia and the United States, uneasy allies on a number of issues including counterterrorism. For years, Russian leader Vladimir Putin sought to portray his military's crackdown on Muslim regions as part of a larger counterterrorism effort. U.S. officials often saw it more as an attempt to suppress human rights and religious freedom.
The Tsarnaev brothers were born to a Chechen family in Kyrgyzstan but had settled in the United States with their parents by 2003. The younger Tsarnaev became a naturalized American citizen in 2012 and Tamerlan Tsarnaev was a legal permanent resident. The parents currently reside in Russia.
In a statement, the FBI said Russia's initial request "stated that it was based on information that [Tsarnaev] was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups.""
In response, the FBI said it interviewed Tsarnaev and his family. Officials said the investigation included examining Tsarnaev's telephone records and visits to extremist websites, reviewing his travel history and education, and analyzing his associations with other persons of interest. The search did not find any links to terrorism but the bureau followed up by requesting more information from the Russian government. The FBI said it did not hear back from Russia, NBC News' Pete Williams reported.
The suspects' mother, Zubeidat Tsarnaeva, confirmed that her son had spoken with the FBI then.
"He was counseled by the FBI for three, five years, they knew what my son was doing" she said. "They knew what actions and what site on the Internet he was going." But her remarks became unclear when she suggested the FBI may have been concerned about him. "They used to come talk to me, they used to tell me that he's really a serious leader, and they are afraid of him."
She said she was certain her sons had been set up and that, although her oldest son Tamerlan was involved in religious politics, "he never told me that he would be on the side of jihad." In an earlier interview, her husband Anzor Tsarnaev insisted that his youngest son was a "true angel."
"He is such an intelligent boy. We expected him to come on holidays here," he said of Dzhokhar.
Dzhokhar was captured after a Watertown resident noticed blood on a backyard boat. He remains in serious condition at Beth Israel Hospital, where 11 victims of Monday's bombing are still recuperating. He could face life in prison but the Justice Department may pursue the death penalty. In addition to federal charges, he will likely also face state charges in the killing of a campus security officer at MIT who was shot and killed in his patrol car late Thursday as the brothers led law enforcement on a high speed chase through the Boston suburbs.
Authorities had yet to read Tsarnaev his Miranda rights citing the controversial public safety rule invoked when there are possible immediate threats.