Larry Ostrowski was an explosives detective in the Big Apple between 1994 and 2000.
He says FBI agents in Boston are looking for a "signature build" in each device, examining what they're made of and how they're assembled.
He says the agents have a lot to go on.
"What do they see at the blast site? What type of materials are remaining at the blast site? An explosive device, when it does detonate, leaves certain undetonated materials," Ostrowski said via phone from Horseshoe Bend.
He believes someone was planting explosives shortly before the marathon began, but ran into large crowds, and had to abandon some of the plan.