"On April 17, 2013, a federal law enforcement source with firsthand knowledge of the investigation told CNN that a lid to a pressure cooker -- thought to have been used in the bombings -- had been found on a roof of a building near the scene. <a href=""http://www.cnn.com/2013/04/17/us/gallery/boston-evidence/index.html"">It was one of several pieces of evidence authorities found</a>."