BOSTON — 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 Friday at the end of a manhunt that shut down Boston.

His brother Tamerlan, 26, was killed in a firefight with police earlier on Friday.

ABC and NBC news networks reported late on Sunday that Dzhokhar Tsarnaev, 19, was awake and responding in writing to questions put to him by authorities after two days under sedation in Boston’s Beth Israel Deaconess Medical Center.

Other U.S. news sources, including CNN, said Tsarnaev, who was shot in the throat and the leg prior to his arrest, was still sedated in the intensive care unit with a breathing tube down his throat.

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, which 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].”

Students returning to campus on Sunday at the University of Massachusetts-Dartmouth, where Dzhokhar Tsarnaev was enrolled, recalled seeing him back in the dorm, at class and even working out in the gym a day or two after the bombings before realizing he was suspected in the crime.

On Saturday, several Republican lawmakers called on the Obama to try Tsarnaev as an “enemy combatant” under terms of war, without entitlement to Miranda rights or appointment of counsel.

“The accused perpetrators of these acts were not common criminals attempting to profit from a criminal enterprise, but terrorists trying to injure, maim, and kill innocent Americans,” Sens. John McCain of Arizona, Lindsey Graham of South Carolina and Kelly Ayotte of New Hampshire, and Rep. Peter King of New York said in a statement.

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, R.I. 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.”

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 Gov. 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.

On Saturday afternoon, the Boston Red Sox returned to Fenway Park for the first time since the bombings, paying an emotional tribute to the victims and the first responders before the game against the Kansas City Royals.

Additional reporting by Martinne Geller, Tabassum Zakaria, Mark Hosenball, John Shiffman, Jim Bourg, Svea Herbst-Bayliss, Daniel Lovering, Ben Berkowitz, Barbara Goldberg, Ed Krudy and Olga Dzyubenko; Writing by Edward Krudy; Editing by Doina Chiacu, Vicki Allen, Peter Cooney and Xavier Briand.