GWEN IFILL: The investigation of the Boston Marathon bombings ramped up today, as police and federal agents pored over the crime scene. Three people are dead, including an eight-year-old boy, and more than 170 others were injured. A handful of those remain in critical condition at various Boston hospitals.
NewsHour correspondent Kwame Holman begins our coverage of the day after.
KWAME HOLMAN: The morning sun filtered across an empty Boylston Street today, littered with the remnants of yesterday's marathon and the double bombings that brought it all to an end.
KEVIN HERVERT, Boston Marathon Runner: Well, it kind of looks like a war zone. It's all cordoned off over here where they have a crime scene. And so it's pretty shocking to know that there's some kind of a terrorist act occurred here and there were bombs right by the finish line that I had just run across.
KWAME HOLMAN: With a new day came new revelations and new perspective on the bombings. This runner wearing a head camera captured the moment the first bomb exploded as she neared the finish line.
MAN: Get out of the stands.
KWAME HOLMAN: More amateur video from a spectator showed the second blast and the ensuing chaos in the bleachers lining the street.
At a briefing this morning, federal, state, and local authorities said all such videos and photographs are vital to their investigation and they asked bystanders to submit whatever they have.
TIMOTHY ALBEN, Massachusetts State Police Superintendent: Hundreds, if not thousands, of photographs or videos or observations that were made down at that finish line yesterday and they're sitting out there amongst everyone that's watching this event this morning. And I would encourage you to bring forward anything. You might not think it's significant, but it might have some value to this investigation.
KWAME HOLMAN: Overnight, the FBI did search an apartment in a Boston suburb, and conspiracy theories grew online, some of them involving a shadowy figure seen yesterday on a rooftop. But for now, officials said they have no suspects and no motive.
Instead, the FBI's special agent in charge said the case is wide open.
RICHARD DESLAURIERS, FBI Special Agent in Charge: This will be a worldwide investigation. We will take -- go where the evidence and the leads take us. We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice.
KWAME HOLMAN: Federal agents also set the record straight on some widely circulated misinformation about the bombings.
GENE MARQUEZ, Bureau of Alcohol, Tobacco, Firearms and Explosives: To dispel any rumors, there were -- there were rumors floating around that there were seven devices -- up to seven devices at one point. That's not true.
I think that happened as a result of some devices -- some suspect packages that were disrupted. But we only have two devices that we're aware of, and both of those devices were the ones that involved in the -- that did the damage and were involved in the explosives incident.
KWAME HOLMAN: Evidence recovered at the scene indicated the bombs possibly were composed of pressure cookers stuffed with shrapnel and left on the ground in black duffel bags. Doctors from local hospitals said the devices were built to do severe bodily harm.
DR. RON WALLS, Chief of Emergency Medicine, Brigham and Women's Hospital: We have removed objects from at least three patients that clearly were designed to be projectiles and were built into the explosive device. These objects are ball bearing-type or small shot-type, just a little larger than BB round metallic beads. They are about two to three millimeters in diameter. And we have also removed over a dozen small carpenter-type nails.
KWAME HOLMAN: Dr. George Velmahos is head of trauma surgery at Massachusetts General Hospital, where 34 victims were taken.
DR. GEORGE VELMAHOS, Massachusetts General Hospital: They have undergone major operations, predominantly, unfortunately, amputations because of the devastating effect of the bombs. Many of them have severe wounds, mostly in the lower part of their body, wounds related to the blast effect of the bomb, as well as small metallic fragments that entered their body, pellets, shrapnel, nails that these bombs had.
KWAME HOLMAN: Nicholas Yanni and his wife, Lee Ann, were standing about 10 feet from where one of the bombs went off. He spoke today from Tufts Medical Center.
NICHOLAS YANNI, Spectator: I think I had a pierced drum, but nothing major. My eardrums are intact and so no major issues on my end. My wife had shrapnel that went through her lower left leg. And I think it shattered her fibula. And they had to do some work with her. And she's got to go in for surgery again tomorrow.
KWAME HOLMAN: Inquiries about the victims lit up social media in the immediate aftermath of the blasts, as did eyewitness posts about the bombings and other information.
And, in Washington today, the flag over the White House was lowered to half-staff in honor of the victims. And the president made a new statement, this time equating the attack with terrorism.
PRESIDENT BARACK OBAMA: This was a heinous and cowardly act. And given what we now know about what took place, the FBI is investigating it as an act of terrorism. Any time bombs are used to target innocent civilians; it is an act of terror.
KWAME HOLMAN: But Mr. Obama said the response proves the attackers will not get their way.
PRESIDENT OBAMA: If you want to know who we are, what America is, how we respond to evil: selflessly, compassionately, unafraid. In the coming days, we will pursue every effort to get to the bottom of what happened and we will continue to remain vigilant.
KWAME HOLMAN: Here at the Capitol, members of Congress put aside partisan differences to mourn the victims and praise the first-responders, and they vowed the nation will have justice and will not give in to terror.
SEN. HARRY REID, D-Nev.: The Federal Bureau Investigation, Department of Homeland Security are investigating this attack as aggressively as possible. As the president said last night, rest assured that the perpetrators will feel the full weight of justice for this terrible crime.
SEN. MITCH MCCONNELL, R-Ky.: For most of us, it's hard to imagine now anyone could even contemplate doing something like this. But, as always, as a nation, we face this sad reality head on and show the world that America doesn't cower in the face of it.
KWAME HOLMAN: The secretary of homeland security, Janet Napolitano, said there's no evidence of a wider plot, but extra security measures will stay in effect for now.
That was apparent across the country with a heightened state of vigilance. Canine teams were out in force at Los Angeles International Airport. A terminal was evacuated at New York's LaGuardia Airport because of a suspicious package. And a plane turned back to the gate at Boston's Logan International Airport because a bag wasn't screened properly.