(McClatchy News Service) Investigators said the two Boston Marathon bombs were triggered by long-range remote controls for toy cars – a more sophisticated design than originally believed – bolstering a theory that the older suspect received bomb-making guidance on his six-month trip to Russia last year.

Tamerlan Tsarnaev, who died in a shootout with police last week, “more than likely got some instruction in Dagestan,” a federal law enforcement official said Wednesday.

The official said investigators continued to believe that Tsarnaev, 26, and his brother, Dzhokhar, 19, were radicalized in the U.S., and that no foreign terrorist group orchestrated the plot.

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