Men in hazmat suits investigate Boston Marathon bombing scene

Two men in haz-mat suits investigate the scene at the first bombing on Boylston Street in Boston Tuesday, April 16, 2013 near the finish line of the 2013 Boston Marathon. Monday two blasts killed at least three and injured over 170 people. <AP Photo/Elise Amendola>

National Guard and Boston police man a barricade in front of the medical tent near the finish line in Boston Tuesday, April 16, 2013 where some of the wounded were treated in yesterday’s explosions at the Boston Marathon. <AP Photo/Winslow Townson>