With the hospitalized Boston bombing suspect unable to speak, attention shifted Sunday to his dead brother, who may have been radicalized or even trained in the Caucasus last year.
US lawmakers questioned why Tamerlan Tsarnaev, 26, killed in a shootout, did not raise more red flags despite being questioned at the request of a foreign government in 2011 and spending six months in the volatile region in 2012.
"Clearly something happened in my judgment in that six-month timeframe... I'm very concerned," Representative Michael McCaul, chairman of the House Homeland Security committee, told CNN's "State of the Union."
"I personally believe that this man received training when he was over there and he radicalized from 2010 to the present."
Republican Senator Lindsey Graham and Democratic Senator Chuck Schumer, appearing on the same show, also expressed concern about the older Tsarnaev.
"It's people like this that you don't want to let out of your sight, and this was a mistake," Graham said about the elder of the two brothers, ethnic Chechens who had been living in the United States for a decade.
"I don't know if our laws are insufficient or the FBI failed, but we're at war with radical Islamists and we need to up our game."
Dzhokhar Tsarnaev, 19, meanwhile remained in a heavily guarded hospital, reportedly unable to speak because of a throat wound suffered in the violent chase that shut down Boston for most of the day Friday.
The FBI would only say Sunday that he was still in "serious condition."
The Tsarnaev brothers are the main suspects in the double bomb attack on the Boston marathon which killed three people and wounded about 180. A policeman was killed and another was seriously wounded in a shootout with the suspects.
Counter-terrorism agents trained in interrogating "high-value" detainees were waiting to question Dzhokhar -- who is being treated at the same hospital as many of the marathon bombing victims -- a law enforcement official told AFP.
Officials have reportedly invoked a "public safety" legal exception that will allow them to question Tsarnaev without reading him his rights.
Graham and fellow Republican Senator John McCain have led calls for the teenager to be declared an "enemy combatant," which would give him the same status as Guantanamo "war on terror" detainees.
Legal rights groups have meanwhile insisted he be given a criminal trial, even though Tsarnaev would likely face the death penalty.
The Tsarnaev family came to the United States from the former Soviet state of Kyrgyzstan around 2002. Dzhokhar became a US citizen in 2011, while his older brother's application was reportedly held up.
Tamerlan began posting militant Islamist videos on social media sites in recent years, and made a six-month trip to Dagestan, which borders Chechnya, last year. Both Russian regions host separatist rebel groups.
The FBI acknowledged on Friday that an unnamed foreign government, reportedly Russia, asked for information about Tamerlan in 2011. The FBI interviewed him but said it had found no "derogatory" information.
The brothers' social media pages appeared to express sympathy with the struggle in Chechnya, which has been ravaged by two wars since 1994 between Russia and increasingly Islamist-leaning separatist rebels.
But a website used by Islamist rebels in Dagestan on Sunday denied any link to the Boston Marathon bombings, saying it was "not waging any military activities against the United States."
"We are only fighting Russia, which is not only responsible for the occupation of the Caucasus, but also for monstrous crimes against Muslims," the Kavkazcenter.com website said.
US President Barack Obama and Russian President Vladimir Putin agreed during telephone talks Saturday to increase cooperation against global terrorism.
Dzhokhar was caught after a man named Dave Henneberry saw blood on boat he kept in his backyard in Watertown, in the Boston suburbs. When Henneberry lifted the tarp he saw the wounded teenager curled up inside, police said.
After law enforcement flooded into the area, thermal images taken by a police helicopter showed Tsarnaev lying the covered boat as a robotic arm lifted the cover so cameras could peek inside.
Over the weekend crowds gathered outside the home of Henneberry -- who has gone into hiding to escape the media blitz -- craning their necks to gaze at his bullet-riddled and blood-smeared 1980s boat.
Fifty-eight of the victims from the bomb attack are still in Boston hospitals, with three in critical condition.