Investigators were seeking a motive for the Boston Marathon bombings and whether others were involved as they awaited a chance on Sunday to interview the surviving ethnic Chechen suspect.
Dzhokhar Tsarnaev, 19, was in a Boston hospital seriously wounded and unable to speak, after he was captured late on Friday at the end of a huge manhunt that shut down Boston.
His brother, Tamerlan, 26, was killed in a firefight with police earlier on Friday.
Investigators are trying to establish whether others may have had a role in the detonation of bombs made in pressure cookers and packed with ball bearings and nails that exploded at the Boston Marathon on Monday, killing three people and injuring 176.
Tamerlan traveled to Moscow in January 2012 and spent six months in the region, a law enforcement source said. But it was unclear what he did while he was there and if he could have had contact with militant Islamist groups in southern Russia's restive Caucasus region.
Authorities have yet to charge Dzhokhar, who will be defended by the Federal Public Defender Office that represents criminal suspects who cannot afford a lawyer.
Sources had suggested he would face charges on Saturday but late in the evening officials from the U.S. Attorneys' Office and the Department of Justice indicated no statements would be issued before Sunday.
The role of the FBI is also being questioned after the agency said it had interviewed Tamerlan in 2011 after Russian security services raised concerns he followed radical Islam. The FBI said it did not find any "terrorism activity" at that time.
But the suspect's mother, Zubeidat Tsarnaeva, who now lives in Russia, told a Russian television station that Tamerlan had been under FBI surveillance for years.
The New York Times, citing unnamed federal officials, reported authorities had held up Tamerlan's application for U.S. citizenship because of the FBI's 2011 interview.
Records show that Tamerlan was arrested when police were called to a report of domestic violence in 2009.
The FBI believes the older brother was the leader of the pair, although investigators were checking on people who had contact with both brothers to see if anyone else was involved, a senior U.S. law enforcement source said.
Early indications are the brothers acted alone, Watertown Police Chief Edward Deveau told CNN on Saturday. "From what I know right now, these two acted together and alone," he said.
Still, the bombings prompted contact between the United States and Russia, and the Kremlin said on Saturday the presidents of both countries agreed by telephone to increase cooperation on counterterrorism.
Ruslan Tsarni, who said he was an uncle of the brothers, told CNN on Saturday he first noticed a change in Tamerlan's religious views in 2009. He suggested the radicalization of his nephew happened while he was in the United States "in the streets of Cambridge (Massachusetts)."
More details of the brothers' seemingly contradictory lives continued to emerge on Saturday.
Tamerlan was married to Katherine Russell, whose family lives in an upper middle-class neighborhood in North Kingstown, Rhode Island. Her father is licensed as an emergency room physician. The couple had a young child.
A statement on the door of the family's home read: "Our daughter has lost her husband today, the father of her child. We cannot begin to comprehend how this horrible tragedy occurred."
'Not Yet Able To Speak'
Dzhokhar, a student at the University of Massachusetts Dartmouth, was shot in the throat and could not speak because of injuries to his tongue, said a source close to the investigation. It was unclear when he would be able to talk.
"We have a million questions and those questions need to be answered," Massachusetts Governor Deval Patrick told reporters on Saturday.
Dzhokhar had been hiding in a boat parked in the backyard of a house in the suburb of Watertown and was captured after a resident spotted blood on the boat and called police. He was being treated at Beth Israel Deaconess Medical Center in Boston.
After combing through a mass of pictures and video from the site in the minutes before the Boston Marathon bombing, the FBI released video of the two men on Thursday and asked the public for help in identifying them.
Just hours later, events began to unfold with the fatal shooting of police officer Sean Collier, 26, on the Massachusetts Institute of Technology campus and finally a massive firefight in Watertown, during which police say the brothers threw bombs at officers. Tamerlan suffered fatal wounds, while Dzhokhar escaped on foot.
The family emigrated to the United States about a decade ago. The brothers spent their early years in a small community of Chechens in the central Asian country of Kyrgyzstan, a mainly Muslim nation of 5.5 million. The family moved in 2001 to Dagestan, a southern Russian province that lies at the heart of a violent Islamist insurgency and where their parents now live.