Lid of Boston Bomb Pressure Cooker Found

May be an important clue

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 of others remain hospitalized from the dual bombings Monday near the finish line of 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 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.