Nightly News   |  April 16, 2013

Millions of pictures, hours of video sent to officials

Investigators have begun the process of recovering tiny pieces of bombs to learn how they were made. So far, they know the bombs were made from pressure cookers filled with ball bearings and nails – a method used for decades in terror bombings. But no suspects are in custody and investigators are asking the public for help. NBC’s Pete Williams reports.

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This content comes from Closed Captioning that was broadcast along with this program.

>> well, good evening from boston , just a few blocks from where the explosions rocked the finish line at the boston marathon yesterday. it was a shocking event in the life of this city. today receiving, of course, widespread coverage as yet another terrorist incident in our post-9/11 era. here's where the toll stands tonight. three dead, 176 injured, 71 people are still hospitalized. 24 of them in critical condition. among the dead, a boston university grad student , an 8-year-old boy, martin richard, from dorchester and 29-year-old krystle campbell. we learned today the president will be coming here on thursday for an interfaith service. at the white house today flags were at half-staff, and we heard again from the president who, for the first time, used the word "terrorism" to describe what happened here.

>> the fbi is investigating it as an act of terrorism. anytime bombs are used to target innocent civilians, it is an act of terror.

>> reporter: the president in the white house briefing room earlier today. and as late as tonight, still no idea as to who did this or why. but we have a better idea as to what was detonated here in boston . nbc's pete williams has been following the investigation in d.c. all day. pete, good evening.

>> brian, good evening. still so many questions, but one has been answered, how many explosive devices were left at the scene. that answer is two. and officials describe them as crudely made but effectively used. and as of tonight, no suspects, no idea who did it or why.

>> reporter: investigators have begun the process of recovering tiny pieces of the bombs to learn how they were made. they say it appears the devices were assembled inside pressure cooker pots similar to this one packed with bbs, ball bearings and nails to magnify the injury. as these homeland security bulletins note, pressure cookers have been used for decades in terror bombings. instructions for making them appear on the internet including just last month in an al qaeda magazine "inspire." investigators say the explosive in boston was most likely smokeless powder, gunpowder like this, available at sporting goods store, not something more powerful like dynamite. and they believe the bombs were carried to the scene in dark nylon bags and set off by timers. the fbi is examining photos like this from nbc affiliate whdh to see if nylon bags might have been placed on trash bags on the sidewalk to appear less conspicuous. a former bomb investigator says the experts know what to look for.

>> they are able to recognize the minute pieces of an recall that a civilian wouldn't even know what it was, but a bomb investigator like fbi or atf will say, that's from a clock. that's from a battery.

>> reporter: boston police say the area was checked twice for bombs yesterday, but there was no security screening .

>> because there is unrestricted access to the race course , simply because it's 28 miles long, people can come and go and bring items in and out.

>> reporter: authorities last night searched the apartment of a foreign student who was injured in the blast and seen running away , but they found nothing, and he is not considered a suspect. investigators urge anyone who was at the scene yesterday to send them pictures and videos. one person responding, ben thorndike, who works in a building that overlooks the finish line . he snapped these pictures in rapid sequence immediately after the first explosion. one person can be seen running away , clothes torn by the blast.

>> i just kept the camera up, just pushed the rapid shutter button down and just took, you know, 25 pictures over the course of -- if felt like a long time, but i think it was only 15 or 20 seconds.

>> reporter: police and federal agents even took the highly unusual step of asking people leaving boston at the airport today if they had pictures of value, too. and this breaking news on another note. an unrelated note. the fbi is analyzing an envelope sent to a u.s. senator but intercepted at an off-site mail site. a field test indicated it was positive for the poison called ricin. it's now in the lab to see if it really is that chemical. brian?

>> reporter: pete williams in our washington bureau tonight. pete, thanks. and