The Boston bombing: Key questions for investigators

Updated
Two men in hazardous materials suits put numbers on the shattered glass and debris as they investigate the scene at the first bombing on Boylston Street in...
Two men in hazardous materials suits put numbers on the shattered glass and debris as they investigate the scene at the first bombing on Boylston Street in...
Elise Amendola/AP

This post was last updated on April 16 at 5:17pm ET.

Investigators know that at around 2:50 p.m. on Monday, essentially the midpoint of the Boston Marathon, two bombs exploded near the finish line, killing at least three people and wounding more than 170. But many other key details remain murky in the investigation, which is currently in its infancy.

Law enforcement officials now believe the bombs appear to have been composed of explosives and shrapnel–BB’s or ball-bearings to increase the damage they would inflict– then packed into pressure cookers that were hidden in dark-colored backpacks. It appears as though these backpacks laced with explosives were carried to the race route, placed near the grandstand and detonated with a timer.

The FBI confirmed Tuesday they have recovered black nylon which could be from a backpack, fragments of BB’s, and nails.

NBC News Correspondent Pete Williams said officials are “optimistic” that within a few days, they will have a “pretty good idea of exactly how these devices were made.”

Boston Police Commissioner Ed Davis told reporters on Tuesday morning that no arrests have been made in the case. Police had questioned a 20-year-old Saudi Arabian student who was among those injured in the blasts. But officials warned it’s still too early to identify suspects or possible motives.

The FBI said there have been no claims of responsibility and the range of suspects and motives “remains wide open.”

Among the media, spectators and participants, there will be hours of video coverage of the event. FBI agents–who are leading the investigation–will scrutinize the available video for any possible clues. They asked the public to send them any photos or video of the Boston Marathon for leads.

The Boston bombing: Key questions for investigators

Updated