4/15: Bombs explode at the Boston Marathon; Coordinated explosions suggest some expertise

      |Two bombs, 12 seconds apart, killed at least two people and injured dozens of others near the finish line of the Boston Marathon. Terrell Brown reports on the chaotic scene following the attack; Also, the bombs were small but powerful explosive devices places along the marathon route. The coordinated explosions suggest some expertise and planning; And, in Los Angeles, California police and sheriffs' departments are providing extra security to high-profile locations, including public transit, government buildings and shopping centers -- places where crowds might gather.

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