Investigators are also casting a wider net for others who may have been associated with the alleged bombers, both in the U.S. and in Russia, where the suspects' parents lived in the southern republic of Dagestan.
The Russian newspaper Novaya Gazeta quoted a Russian law enforcement source as saying that during a six-month visit to Dagestan in 2012, Tamerlan Tsarnaev was seen "more than once" meeting with Makhmud Mansur Nidal, suspected of being part of a rebel group that previously had planted two bombs in the Dagestani capital of Makhachkala.
Nidal, who was of joint Palestinian and Dagestani extraction, was killed May 19, 2012, when Russian authorities laid siege to a house where he and six other suspected insurgents were holed up, a Dagestani Interior Ministry official told the Los Angeles Times.
Novaya Gazeta reported that Russian agents also discovered that Tsarnaev had some online communications as early as 2010 with William Plotnikov, then 21, a Canadian citizen and Russian native who, like Tsarnaev, was a boxer. He had been living in the Toronto area with his family since 2005.
Plotnikov traveled to Dagestan purportedly to study Islam, but was one of eight people identified as insurgents killed by law enforcement forces on July 14, 2012, in one of Dagestan's remote mountain villages. Two days later, Tsarnaev left Dagestan for Moscow; a day after that, he left for the U.S., without waiting to receive the new passport that Russian officials have said was one of the purposes of his trip.