The pressure cookers used in the Boston marathon blasts had no brand name but had marking '6L'. 6 L indicates six litres and the pressure cookers in the United States are mostly marked in quarts, not litres. The litre measurement is used mostly in Asia and South-east Asia.
The pressure bombs are easy to make and can maximize injuries and fatalities. They have a tight-fitting lid, which allows bomb-makers to pack shrapnel and explosive material closely, maximizing its acceleration after an explosion and cause maximum injuries.
The bombs are filled with explosive material like gunpowder, metal items like ball bearings or nails and a detonator that can be remotely triggered.
The pressure cooker bombs were used in the Mumbai train blasts of 2006 that killed more than 100 people, blast in Varanasi in 2006, blasts in Delhi's Sarojini Nagar in 2005, killing dozens.
Internet is full of instructions on how to make IED (improvised explosive device) and turn the pressure cooker into a bomb.
The Department of Homeland Security had warned in 2004 that terrorists were turning pressure cookers and other homemade bombs. But the alert has failed the people of Boston.
Meanwhile, the lid of a pressure cooker thought to have been used in the bombings has been found on a roof of a building at the scene possibly yielding vital clues.
One of the bombs was housed in a pressure cooker hidden inside a backpack, the Federal Bureau of Investigation (FBI) said in a Joint Intelligence Bulletin.
The device also had fragments that may have included nails and ball bearings, the agency said. The second bomb was also housed in a metal container, but there's not enough evidence to determine whether it was also in a pressure cooker, the FBI said.
Scraps of at least one pressure cooker, nails and nylon bags found at the scene are being sent to the FBI's national laboratory at Quantico, Virginia, where technicians will try to reconstruct the devices.
The pieces recovered so far suggest the devices could carry 6 litres each. The parts found also include a partial circuit board, which would be used to detonate a device, as well as the lid found on the rooftop.
The investigators are also sifting through more than 2,000 tips and a mass of digital photos and video clips, but are asking for the public's help in providing additional leads and images.
With inputs from agencies.