The FBI had interviewed and investigated Tamerlan Tsarnaev, the dead Boston bomber, in 2011 and cleared him of any terrorist link.
Tamerlan was put on the FBI radar by Russia, with a request for information about him to process his papers for travel to the disturbed regions of Chechnya and Dagestan.
Investigators are taking a fresh look at this trip to find any evidence of Tamerlan's radicalisation. The FBI said in a statement on Friday that a "foreign government" had asked about Tamerlan saying that he was travelling to Chechnya and Dagestan to join "unspecified underground groups".
That "government" has since been identified as Russia. And Presidents Barack Obama and Vladimir Putin spoke late Friday night about the need for closer cooperation against terrorism.
Acting on Moscow's request the FBI had questioned Tamerlan and several of his family members. It also looked for indications of his radicalisation - "derogatory" calls, online postings.
But it "did not find any terrorism activity, domestic or foreign". It said it asked for more and specific information, but didn't hear back from the Russians. The bureau also didn't keep track of Tamerlan after that. But now the investigators are saying that they found signs of his radicalisation, according to reports, after his return in 2012.
"One month after he returned to the United States, a YouTube page that appeared to belong to him was created and featured multiple jihadist videos that he had endorsed in the past six months," said a New York Times report.
North Caucasus rebels deny link
A website used by Russia's North Caucasus rebels has denied any link to the Boston bombings. "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 Kavkazcenter.com website said.