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      Boston Marathon runners and families lay flowers near finish line

      Runners, neighbors, and families haven created a memorial two blocks from Boston Marathon finish line.

      Authorities are gradually opening some streets in Boston??s Copley Square where two bombs exploded at the Boston Marathon on Monday. Areas closer to the crime scene remain closed Wednesday morning as authorities continue to look for evidence.

      Runners, neighbors, and families of those injured or killed in the explosions haven been leaving flags, flowers and messages at the intersection of Boylston Street and Berkley Street, two blocks from Boston Marathon finish line.

      As runners looked at the memorial, they could see investigators sweeping the Marathon route for evidence.

      (Watch: Raw video of the crime scene investigation.)

      Several runners said it was difficult to look at the finish line as an active crime scene, but say they are already planning on coming back next year.

      Officials say more than 170 people were injured and three were killed in the explosions Monday afternoon. President Barack Obama said Tuesday that the explosions are being investigated as an act of terror, although authorities still don't know who is responsible.