Forensic clues are starting to emerge from the aftermath of the Boston Marathon bombing but there is still no evidence as to who was behind the deadly attack.
The double bombing, which killed three people and injured more than 170 others, was the worst attack in the United States since September 11th.
Clues point to the use of a pressure cooker to trigger the devices. Investigators believe the suspect or suspects carried heavy bags or backpacks. They have also recovered pieces of black nylon, ball bearings and nails. All of which could prove crucial to finding the killers.