Ryan Borker, a runner who had an aerial view of the finish line during the explosions, and Kate Fox, a spectator who was in a bar, both saw the Boston Marathon bombings and frantically ran to safety. They told Anderson Cooper what it was like in the aftermath of the attack.

Former FBI Assistant Director Thomas Fuentes and CNN National Security Analyst Fran Townsend explain what evidence investigators could use to find who's responsible for the Boston bombings.

If you have information about the attack, call the Boston crime tips hotline at 800-494-TIPS.

Capt. Thom Kenney is an Afghan war veteran who ran the Boston Marathon and finished the race minutes before the bomb attack. He describes what he witnessed to Anderson Cooper.

James Brennan says he was just five feet away from where the bombs went off at the Boston Marathon finish line. His was ringing up customers in his candy store when chaos erupted. He describes the scene to Anderson Cooper as "absolutely heart-wrenching."