Previously officials said Tsarnaev was in no condition to be interrogated.
Sources tell ABC News the suspect has a wound to the neck and throat area that has made it difficult for him to speak.
It is still unclear when he will officially be able to talk, and whether he will be willing to talk.
Meanwhile the American Civil Liberties Union and a federal public defender raised concerns about investigators' plan to question 19-year-old Dzhokhar Tsarnaev without reading him his Miranda rights.
Tsarnaev remained in serious condition and apparently in no shape for interrogation after being pulled bloody and wounded from a tarp-covered boat in a Watertown backyard.
The capture came at the end of a tense Friday that began with his 26-year-old brother, Tamerlan, dying in a shootout with police.
U.S. officials said an elite interrogation team would question the Massachusetts college student without reading him his Miranda rights, something that is allowed on a limited basis when the public may be in immediate danger, such as instances in which bombs are planted and ready to go off. ACLU Executive Director Anthony Romero said the legal exception applies only when there is a continued threat to public safety and is "not an open-ended exception" to the Miranda rule, which guarantees the right to remain silent and the right to an attorney.
The federal public defender's office in Massachusetts said it has agreed to represent Tsarnaev once he is charged. Miriam Conrad, public defender for Massachusetts, said he should have a lawyer appointed as soon as possible because there are "serious issues regarding possible interrogation."
After the two brothers engaged in a gun battle with police early Friday, authorities found many unexploded homemade bombs at the scene, along with more than 250 rounds of ammunition.
Police Commissioner Ed Davis said the stockpile was "as dangerous as it gets in urban policing."
"We have reason to believe, based upon the evidence that was found at that scene - the explosions, the explosive ordnance that was unexploded and the firepower that they had - that they were going to attack other individuals. That's my belief at this point." Davis said.
The suspects in the twin bombings that killed three people and wounded more than 180 are two ethnic Chechen brothers from southern Russia - 19-year-old Dzhokhar Tsarnaev and his 26-year-old brother, Tamerlan. Their motive remained unclear.
The older brother was killed during a getaway attempt. The younger brother, Dzhokhar Tsarnaev, was still in serious condition Sunday after his capture Friday from a tarp-covered boat in a suburban Boston backyard. Authorities would not comment on whether he had been questioned.
Sen. Dan Coats of Indiana, a member of the Senate Intelligence Committee, said Tsarnaev's throat wound raised questions about when he will be able to talk again, if ever.
The wound "doesn't mean he can't communicate, but right now I think he's in a condition where we can't get any information from him at all," Coats told ABC's "This Week."
There was no immediate word on when Tsarnaev might be charged and what those charges would be. The twin bombings killed three people and wounded more than 180.
The most serious charge available to federal prosecutors would be the use of a weapon of mass destruction to kill people, which carries a possible death sentence. Massachusetts does not have the death penalty.
President Barack Obama said there are many unanswered questions about the bombing, including whether the Tsarnaev brothers - ethnic Chechens from southern Russia who had been in the U.S. for about a decade and lived in the Boston area - had help from others. The president urged people not to rush judgment about their motivations.
Gov. Deval Patrick said Saturday afternoon that Tsarnaev was in serious but stable condition and was probably unable to communicate. Tsarnaev was at Boston's Beth Israel Deaconess Medical Center, where 11 victims of the bombing were still being treated.
"I, and I think all of the law enforcement officials, are hoping for a host of reasons the suspect survives," the governor said after a ceremony at Fenway Park to honor the victims and survivors of the attack. "We have a million questions, and those questions need to be answered."
The all-day manhunt Friday brought the Boston area to a near standstill and put people on edge across the metropolitan area.
The break came around nightfall when a homeowner in Watertown saw blood on his boat, pulled back the tarp and saw a bloody Tsarnaev hiding inside, police said. After an exchange of gunfire, he was seized and taken away in an ambulance.
Raucous celebrations erupted in and around Boston, with chants of "USA! USA!" Residents flooded the streets in relief four days after the two pressure-cooker bombs packed with nails and other shrapnel went off.
Michael Spellman said he bought tickets to Saturday's Red Sox game at Fenway Park to help send a message to the bombers.
"They're not going to stop us from doing things we love to do," he said, sitting a few rows behind home plate. "We're not going to live in fear."
During the long night of violence leading up to the capture, the Tsarnaev brothers killed an MIT police officer, severely wounded another lawman and took part in a furious shootout and car chase in which they hurled explosives at police from a large homemade arsenal, authorities said.
Watertown Police Chief Edward Deveau said one of the explosives was the same type used during the Boston Marathon attack, and authorities later recovered a pressure cooker lid that had embedded in a car down the street. He said the suspects also tossed two grenades before Tamerlan ran out of ammunition and police tackled him.
But while handcuffing him, officers had to dive out of the way as Dzhokhar drove the carjacked Mercedes at them, Deveau said. The SUV dragged Tamerlan's body down the block, he said. Police initially tracked the escaped suspect by a blood trail he left behind a house after abandoning the Mercedes, negotiating his surrender hours later in the boat.
Chechnya, where the Tsarnaev family has roots, has been the scene of two wars between Russian forces and separatists since 1994. That spawned an Islamic insurgency that has carried out deadly bombings in Russia and the region, although not in the West.
Investigators have not offered a motive for the Boston attack. But in interviews with officials and those who knew the Tsarnaevs, a picture has emerged of the older one as someone embittered toward the U.S., increasingly vehement in his Muslim faith and influential over his younger brother.
The Russian FSB intelligence service told the FBI in 2011 about information that Tamerlan Tsarnaev was a follower of radical Islam, two law enforcement officials said Saturday.
According to an FBI news release, a foreign government said that Tamerlan Tsarnaev appeared to be strong believer and that he had changed drastically since 2010 as he prepared to leave the U.S. for travel to the Russian region to join unspecified underground groups.
The FBI did not name the foreign government, but the two officials said it was Russia. The officials spoke on condition of anonymity because they were not authorized to talk about the matter publicly.
The FBI said that in response, it interviewed Tamerlan Tsarnaev and relatives, and did not find any domestic or foreign terrorism activity. The bureau said it looked into such things as his telephone and online activity, his travels and his associations with others.
An uncle of the Tsarnaev brothers said he had a falling-out with Tamerlan over the man's increased commitment to Islam.
Ruslan Tsarni of Montgomery Village, Md., said Tamerlan told him in a 2009 phone conversation that he had chosen "God's business" over work or school. Tsarni said he then contacted a family friend who told him Tsarnaev had been influenced by a recent convert to Islam.
Tsarni said his relationship with his nephew basically ended after that call.
As for Dzhokhar Tsarnaev, "he's been absolutely wasted by his older brother. I mean, he used him. He used him for whatever he's done," Tsarni said.
Albrecht Ammon, a downstairs-apartment neighbor of Tamerlan Tsarnaev in Cambridge, said in an interview that the older brother had strong political views about the U.S. Ammon quoted Tsarnaev as saying that the U.S. uses the Bible as "an excuse for invading other countries."
Tamerlan Tsarnaev studied accounting as a part-time student at Bunker Hill Community College in Boston for three semesters from 2006 to 2008, the school said. He was married with a young daughter. Dzhokhar Tsarnaev was a student at the University of Massachusetts Dartmouth.
As of Saturday, more than 50 victims of the bombing remained hospitalized, three in critical condition.
The slain MIT officer was identified as 26-year-old Sean Collier, of Somerville. Sources say he was ambushed, killed in cold blood while he was sitting in his car. CLICK HERE for more on Sean Collier.
Pictures from the Boston Marathon bombings (The Associated Press contributed to this report)