With the hospitalised 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 radicals 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."

Meanwhile, a website used by Russia's North Caucasus rebels denied Sunday any link to the deadly Boston Marathon bombings that have been blamed on two ethnic-Chechen suspects.

"The command of the Vilayat Dagestan mujahedeen... declares that the Caucasus fighters are not waging any military activities against the United States of America," the website said.

"We are only fighting Russia, which is not only responsible for the occupation of the Caucasus, but also for monstrous crimes against Muslims," the rebel site said.

US media reports said the FBI was studying possible links between the two suspects - brothers Dzhokhar and Tamerlan Tsarnaev - to the Caucasus Emirate movement led by feared warlord Doku Umarov.