The older brother in the Boston bombing case returned from a six-month stay in Russia last year "with a willingness to kill people", John Kerry said yesterday.
Speaking in Brussels, the US secretary of state said: "We just had a young person who went to Russia, Chechnya, who blew people up in Boston.
"So he didn't say where he went, but he learned something where he went and he came back with a willingness to kill people."
The US State Department later clarified Mr Kerry's remarks, saying he was "simply expressing broad concern about radicalism rather than indicating any new information".
US officials have not determined if 26-year-old Tamerlan Tsarnaev, who died last Friday in a police shootout, became radicalised in Russia.
The FBI has already admitted it interviewed Tsarnaev in 2011 after Russia raised concerns that he was becoming a follower of radical Islam, but found nothing "derogatory" against him and did not pursue the case.
Russian security services monitored Tsarnaev during the five-month visit he made to the North Caucasus region of Dagestan in 2011-2012, but found no reason to arrest him, a security source said earlier this week.
Mr Kerry's remarks came amid claims that Tsarnaev reportedly fell under the sway of a mysterious Muslim convert who steered him to a severe brand of Islam.
Tsarnaev was reportedly deeply influenced by the figure, identified only as "Misha", an Armenian he met in Cambridge, Massachusetts, about four years ago.
During the course of several years under Misha's guidance, Tsarnaev is said to have grown increasingly radical and turned to al-Qaeda-linked websites.
"Somehow, he just took his (Tamerlan's) brain," said Ruslan Tsarni, his uncle.
Tamerlan is also reported to have appeared on a US watch list of potential terrorists but was not tracked because the list was too large.
He was one of more than 540,000 names on a classified database but did not automatically qualify for close surveillance, according to Reuters.
US investigators have interviewed the suspects' parents near their home in the Russian republic of Dagestan.
Dzhokhar Tsarnaev (19) Tamerlan's brother, is in hospital with gunshot wounds. He has been charged with the bomb attack on the Boston marathon. US officials yesterday said that the bombs were triggered by a remote-controlled detonator. (© Daily Telegraph, London)