The FBI says the two bombs that exploded Monday at the Boston Marathon were carried in a dark-colored bag, and possibly consisted of pressure cookers packed with nails and ball bearings.
FBI examiners are looking at fragments recovered from the scene, as they try to piece together how the attack happened.
Authorities do not have any suspects, and are asking the public to send in any photos or videos taken along the marathon route to help find who was responsible.
VOA correspondent Carolyn Presutti, reporting in Boston, says police are also asking people to call in any possible tips.
Police and firefighters unions in Boston have offered a $50,000 reward for information leading to an arrest.
The FBI agent in charge of the case says investigators will "go to the ends of the Earth" to identify who carried out the bombings.
Maryland Congressman Dutch Ruppersberger said after a security briefing Tuesday it is not likely that al-Qaida or any foreign government was involved because of the lack of prior intelligence.
Two blasts seconds apart killed three people at the finish line of the Boston Marathon, including an 8-year-old boy. One-hundred-76 people were injured. A number of them lost limbs.