The twin bombs that exploded at the Boston Marathon were likely carried to the scene in dark nylon bags and fashioned out of pressure cookers, authorities have stated.

Both bombs were likely built using pressure cookers as the superstructure, black powder or gunpowder as the explosive and ball bearings as additional shrapnel, according to experts.

The bombs are said to be similar to the improvised explosive devices found in war-torn regions like Iraq and Afghanistan. However, information on how to make the bombs is also believed to be readily found online.

A 2010 bulletin issued by the Department of Homeland Security and the FBI warned pressure-cooker bombs are as easy to transport as they are to conceal.

Three people were killed in the two bomb explosions at the finish line of the Boston Marathon on Monday. The explosions occurred at about 2:45 p.m. Eastern Time, more than two hours after the first of the race's nearly 27 000 runners had crossed the finish line.

US President Barack Obama has described the bombing at the Boston Marathon as an "act of terror", adding that it was yet to be revealed whether it had been plotted by a foreign or domestic group or individual.