CNN -- Within a day of the Oklahoma City bombing, officials had named their suspect: Timothy McVeigh. Within two days of the 9/11 attacks, investigators had zeroed in on al Qaeda as the perpetrator.
But as loved ones mourn the deaths of three people and dozens remain hospitalized from dual bombings at the Boston Marathon, two questions continue to hound authorities: Who triggered the attack, and why?
On Wednesday morning, a federal law enforcement source with firsthand knowledge of the investigation told CNN's Fran Townsend that a lid to a pressure cooker thought to have been used in the bombings had been found on a roof of a building at the scene.
Read the story here.