NBC News   |  April 15, 2013

Boston's proudest day turns into nightmare

NBC News' Dennis Murphy reports on how the day of the 2013 Boston Marathon unfolded as its traditionally proudest day into an unimaginable 'act of terrorism' that has ramped up investigations in Boston and security measures in major cities around the world.

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

a plume of thick, gray smoke spiraled up over boylston street marking the two spots virtually the finish line where who knew what had just happened.

>> multiple people down here. okay? i don't know what the cause is. stand by.

>> reporter: but whatever it was, a great many people have suffered ghastly injuries. both runners and spectators cheering on the back of the pack finishers four hours after the start of the marathon. they were rushed in agony to a medical tent set up to treat not more than dehydration and exhaustion.

>> this is like a bomb explosion that we hear about in the news in baghdad or israel.

>> reporter: witnesses reported seeing severed legs and pools of blood. competitors crossed their finish line on stretchers, in wheelchairs. boston 's proudest day had been tarnished in unimaginable fashion.

>> people's limbs have been blown off and all i knew what to do is just pray.

>> reporter: authorities would soon confirm what most people expected but didn't want to hear. this wasn't a gas leak . someone had bombed the boston marathon .

>> we are asking everybody to stay out of the perimeter of the explosions, about a 15-block area which is secured by the state police . that is a crime scene right now.

>> reporter: chaos. ambulance teams loading the injured. citizens tending to the fallen as best they could. here on the final mile of a race that had been dedicated to the murdered children, grade school in newtown.

>> at first it sounded like a cannon blast and felt so strong it almost blew my hat off. and everybody started running the other way. there were a few people helping the injuries and bad, bad injuries. some people were badly hurt.

>> carnage. just carnage. leg. and then all of the folks from the marathon treating the runners and called in to action.

>> reporter: there was confusion. reports of a device exploded simultaneously miles away at the kennedy presidential library . that was not true. reports police bomb squads finding other unexploded devices in the area. reports that the authorities ordered boston cell phone service shut down lest the bomber or bombers detonate more explosions by remote control. the rumor was untrue. the phones stayed up. bosston bost bostonians advised to stay inside until the big picture cleared. was this an ongoing attack? no one knew. in cities across the country, los angeles to new york, security and vigilance were stepped up. in washington, the department of defense added a bigger uniform presence around the white house and the pentagon. president obama said today those responsible will feel the full weight of justice.

>> we will get to the bottom of this and find out who did this, we'll find out why they did this. any responsible individuals, any responsible groups will feel the full weight of justice.

>> reporter: this evening in boston , authorities offered few new details.

>> the fbi is taking the lead in this investigation. it is a criminal investigation that is a potential terrorist investigation.

>> reporter: the marathon set off this morning in almost perfect conditions. temperatures around 50. comfortable racing conditions for the 24,500 runners bibbed and ready to compete for glory, personal best or the honor of it. in boston , it's patriots day and usually a joyous day off for hundreds of thousands of spectators. the predictor of spring's arrival in new england as surely as the return of the robins. today, someone, a person or persons unknown , stole all of that. killing and maiming innocents who can now never forget the horrific day they ran when the finish line was strung with crime scene tape .