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Rep. Adam B. Schiff, California Democrat and member of the House intelligence panel, said it’s “probably true” that a major terrorist group wasn’t involve in the bombings. But he said investigators have much to learn about the older brother’s visit to the Russian region of Dagestan last year.

“There may have been radicalizing influences in the United States. There may have been radicalizing influences overseas,” Mr. Schiff said Sunday on CNN’s “State of the Union.”

Russian authorities warned the United States in recent years that Tamerlan Tsarnaev was a security threat, though U.S. officials have complained that, after initially checking out the tips, Moscow failed to cooperate.

“There is a lot that we just don’t know, and that’s why many say, hey, the Russians need to step up to the plate here and provide us better information,” Mr. Rogers said. “I think they have information that would be incredibly helpful that they haven’t provided yet.”