Investigators analyzing debris from Boston bombs

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(CNN) –

The explosions

— A lid to a pressure cooker, thought to have been used in the bombings, was found on a roof near the scene of the blasts, a federal law enforcement official with knowledge of the investigation tells CNN.

— The two bombs hit 12 seconds apart near the marathon’s finish line.

— One of the two bombs was housed in a pressure cooker hidden inside a backpack, the FBI said. To maximize the impact of the shrapnel, the device may have included “nails, BBs and ball bearings.”

— The second bomb was also in a metal container, but there’s not enough evidence to determine whether it too was in a pressure cooker, the agency said.

— Evidence recovered from the scene will be sent to the FBI’s lab in Quantico, Virginia, where analysts will attempt to reconstruct the devices used in the attack.

Pressure cooker bombs

— The simplicity of the bomb makes it hard to trace it to any particular group, an official said.

— The “recipe” for the bombs ignited by pressure cookers can be found widely on the Internet.

— In 2004, Homeland Security issued an advisory about pressure cooker bombs.

— They are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top, the advisory said. Pressure cooker bombs are made with readily available materials.

— Though it could indicate domestic terrorism, such bombs have been used in a handful of instances related to international terrorism attempts over the last few years, an official said.

Terror group links

— No connection has been made to any terrorist group or individual.

— “There is no reporting indicating a foreign connection, or any reaction from al Qaeda,” a senior U.S. official said.

— President Barack Obama described the bombings as an act of terrorism, but said it is unclear whether they were the work of a group or “a malevolent individual.”

— The Pakistani Taliban has said it was not involved in the attack.

— Authorities don’t have a sense of what the motive is, and no one is in custody, an official said.

— Richard DesLauriers, the special agent in charge of the FBI’s Boston office, asked the public to report anyone who talked about targeting the marathon or showed interest in explosives. He urged anyone who might have heard explosions in remote areas — possibly by someone testing a bomb — or seen someone carrying “an unusually heavy, dark-colored bag” around the time of the attack to come forward.