Bombs were designed to spew destruction outward ← Related Article Visit The Boston Globe Share on Twitter Share on Facebook Comment on this Scroll to top of page AP Photo/FBI This image from the Federal Bureau of Investigation and Department of Homeland Security shows the remains of a pressure cooker that the FBI says was part of one of the bombs that exploded during the Boston Marathon. Reuters Boston Marathon bomb scene pictures taken by investigators show the remains of an explosive device. AP Photo/FBI The remains of a black backpack that the FBI says contained one of the bombs that exploded during the Boston Marathon. Reuters Many questions remain about the chemical makeup of the explosives within the bomb and exactly how they were triggered, but on Tuesday investigators announced some progress. Reuters Boston Marathon bomb scene pictures taken by investigators show the remains of an explosive device. Reuters Investigators recovered parts of the bombs, enabling them to determine they were constructed out of two 6-liter pressure cookers packed with nails, metal, and ball bearings. Reuters Debris from the explosion was found on rooftops, giving a sense of the bomb's force. Reuters By looking at damage to a structure or the types of injuries, it is possible to reconstruct aspects of the device. Reuters Investigators are working with fragments painstakingly gathered at what is considered the city’s largest-ever crime scene. Reuters The FBI asked the public for tips on anyone who may have been lugging a heavy black bag near the finish line. Reuters The bomb-making technique used in the attack was detailed in a 2010 article in the online magazine, Inspire, published by the terrorist group Al Qaeda. Reuters A pressure cooker bomb “is not a high-tech device,” said Michael White, director of explosives and training with MSA Security. Reuters The bombs detonated 12 seconds apart just after 2:50 p.m. Monday.