Federal prosecutors were putting the finishing touches on charges against the surviving marathon bombing suspect on Sunday - as NBC News learned that Russian intelligence agencies contacted the FBI last year about his older brother.
Dzhokhar Tsarnaev, the 19-year-old man accused in the Boston Marathon bombing that killed three people and injured more than 180 remained in serious condition at Beth Israel Deaconess Medical Center.
Law enforcement sources said federal prosecutors are putting the final touches on the charges against Tsarnaev, and their goal is to file them today.
A senior law enforcement official also confirmed Sunday that Dzhokhar Tsarnaev is suffering from a wound to his throat that has left him unable to talk.
This has delayed efforts by the special high value detainee interrogation team to talk to him without advising him of his Miranda rights, the source said. Gov. Deval Patrick told reporters Saturday that Tsarnaev was "unable to communicate."
Boston Mayor Thomas Menino said investigators may never be able to orally question Dzhokhar Tsarnaev in an interview on Sunday. "And we don't know if we'll ever be able to question the individual," Menino said.
Dzhokhar Tsarnaev remained in "serious condition," the FBI said in a statement released on Sunday at the request of the hospital where a number of bombing victims also have received treatment.
His brother, the man identified by the FBI as Suspect 1, 26-year-old Tamerlan Tsarnaev, was killed in a firefight with police following a wild chase into the suburb of Watertown on Thursday night.
The brothers hurled a pressure-cooker bomb similar to the two that went off at the marathon during the firefight, Watertown Police Chief Edward Deveau said on Saturday. The men were in two cars when confronted by a lone police officer, Deveau said, and later threw four grenade-like explosives at pursuing officers.
Much remained unknown on Sunday about what might have driven the two suspects to violence.
A police official source in Makhachkala, Dagestan, told NBC News on Sunday that the Russian internal security service reached out to the FBI last November with some questions about Tamerlan, and handed over a copy of case file on him.
Tsarnaev had first popped up on the local police radar in Dagestan last summer, the source said. During routine surveillance of an individual known to be involved in the militant Islamic underground movement, the police witnessed Tamerlan meet the latter at a Salafi mosque in Makhachkala, the police official said.
It was one of six times in total that surveillance officials witnessed Tsarnaev meeting this militant at the same mosque, according to the police official.
The militant contact later disappeared, the police official said, but so did Tsarnaev before investigators had a chance to speak with him. The FBI never responded, according to the Dagestani police official.
Also, crime scene units returned to the scene of Monday's twin explosions that brought an annual springtime rite to an end in screams and smoke. Debris and trash not far from the bomb site on Boylston Street were taken away in garbage trucks on Sunday after being sifted for evidence.
Secretary of Defense Chuck Hagel said he has not seen evidence to link the bombings to any militant or terrorist group on Sunday, and declined to speculate on whether or not Dzhokhar Tsarnaev could be sent to Guantanamo Bay.
"We just don't have the facts, and until we get the facts, then it will be the responsibility of law enforcement, DOJ, and other institutions to make some determination as to how that individual should be treated, detained, charged, and all that goes with it," Hagel said. "But right now we just don't know enough about it."
A funeral for marathon victim Krystle Campbell, 29, a restaurant manager, is scheduled for Monday at St. Joseph Church in her home town of Medford, Mass.
About 36,000 runners participated in the London Marathon on Sunday amid heightened security, many of them wearing black ribbons to commemorate the victims in Boston or carrying "For Boston" signs.
NBC News' Tom Winter, Michael Isikoff and Michael Brunker contributed to this report.