Officials are calling the site of the Boston bombing the most complex crime scene in the history of the city's police department.
A person briefed by the FBI said the explosives were inside 6-liter pressure cookers and were placed in black duffel bags.
The source said the explosive devices were placed on the ground and contained shards of metal, nails and ball bearings. Law enforcement officials have some of the bomb components but did not yet know what was used to set off the explosives. And no one has claimed responsibility, so far.
Three have died, including an 8-year old boy. Another 170 people are wounded. Seventeen of those are in critical condition. A doctor at Boston Children's Hospital said they removed BB pellets and nails from the bodies of the underage patients.
FBI Special Agent Richard Deslauriers said, "There are no known additional threats and agents are following a number of leads."
Officials say two bombs went off at the site of the marathon but they did not find any other unexploded devices."Rumors of seven devices but that was not true, we have only have two devices and there are the only two used in the incident," said Gene Marquez, from The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).
Officials are looking over all the surveillance video and photographic evidence. Some say this is probably one of the most photographed areas in the country.
To date several "persons of interest" are being investigated according to local officials. They include a young Saudi national, who is being treated at a local hospital, and another person who was seen trying to get into a restricted area just before the bombs went off.
Massachusetts State Police also searched an apartment in a suburb of Boston on Monday. Police say the search was related to the investigation but gave no other details. "I cannot say who may or may not be in custody right now," said Deslauriers.
Experts say the lack of damage to buildings may suggest the bombs may not have worked as intended. "We would have a much higher casualty rate and injury rate," said Kevin Barry Explosive Expert.