BOSTON ON HIGH ALERT AFTER 3 KILLED, 140 INJURED IN MARATHON BOMBING

By Bill Dedman, Alastair Jamieson, Tracy Connor and Erin McClam, NBC News

Authorities checked bags, combed through debris and flooded the streets with investigators Tuesday in a rush to determine who set off two shrapnel-studded bombs at the Boston Marathon that killed three people and injured at least 144.

Law enforcement officials went to a suburban apartment building, interviewed two men and removed a duffel bag and two trash bags overnight. They also issued alerts for a rental van and a man seen leaving the scene of the blasts.
But the significance of the police activity, if any, was not clear one day after the twin blasts, which detonated seconds apart and turned the finish line of the marathon into a hellish scene of panicked spectators, shattered glass and blood-spattered sidewalks.

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Law enforcement officials told NBC News that the bombs were packed with ball bearings and BBs, apparently intended to increase the casualties.

Among the dead was an 8-year-old boy, identified by NBC affiliate WHDH as Martin Richard, who was waiting at the finish for his father to finish the race. Among the injured were brothers, 33 and 31, who each lost a leg from the knee down, The Boston Globe reported.

As of 4:30 a.m. ET, there were 126 patients from the blasts being treated at six Boston hospitals, including some who had injuries described as “limb-threatening.” An official at Boston Children’s Hospital told the Globe that the youngest victim was 3 years old.

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A senior law enforcement official told NBC News that the attack involved two bombs. Initial reports had suggested that there were unexploded devices found nearby, but the official discounted those reports.

Investigators were also studying surveillance video to look for anyone placing packages at the points where the bombs exploded. Law enforcement officials said there was video showing a person, from the back, carrying two backpacks, but they said it was too soon to know whether that was related to the attack.

Long after nightfall, investigators picked through mounds of personal belongings left by some of the thousands of people who fled the finish line, at Boylston Street, after gathering for perhaps the most joyous day on the Boston calendar — Patriots Day.

“Patriots Day and Marathon Monday are usually celebrated all across Boston, but today our cheers were turned to prayers,” Mayor Thomas Menino said. “But Boston is strong and resilient, and we come together in times of need. We will get through this, and we will find those responsible.”

Firefighters spent the night chasing bomb scares around a jittery city, and SWAT officers patrolled nearby hotels. Water cannons were used late Monday to clear suspicious packages near the scene.

A fire at the John F. Kennedy presidential library more than an hour after the blasts, initially treated by investigators as related, appeared be caused by a simple electrical short, police said.

The blasts went off just before 3 p.m., with thousands of runners still on the course. They sent up white plumes of smoke, rattled police barricades and stiffened flags set up to celebrate the 96 countries that sent runners to the most prestigious road race.

“In 28 years, this is definitely the worst I’ve seen,” said Chief Ron Harrington of the Boston Fire Department’s District 3. “Bodies and body parts. Blood all over. A little boy lying in the street. A young woman in her 20s. Both dead. It was mayhem. I saw two people with arms hanging loose, and one without a leg. A shoe with flesh still in it.”

President Barack Obama vowed from the White House to bring “the full weight of justice” to bear on whoever was responsible, and pledged the help of the federal government.

The Boston Globe's Steve Silva talks to NBC's Brian Williams about the footage he shot at the moment explosions rocked the finish line of Monday's Boston Marathon.

A White House official told NBC News that the attack was being treated as terrorism, though it was not clear whether foreign or domestic.

Meanwhile, authorities in other big cities increased security. Extra police officers patrolled the New York subways, and streets were closed around the White House. In London, officials preparing for an upcoming marathon reviewed security plans.

NBC News national security analyst Michael Leiter said it was “virtually impossible” to keep a marathon secure because of its long route.

“You just have to do the best you can to keep people safe and maintain resilience,” he said. “It’s important we don’t alter our lives because that provides the terrorist — domestic, international, whoever it may be — with a huge victory.”

The apartment that investigators went to was at 364 Ocean Ave., in the Boston suburb of Revere. Law enforcement officials interviewed two men there.

Boston police issued an alert for a rental van that may have sought access to the marathon route, and another alert for a man wearing dark clothes and a hood who was seeing leaving the blast zone.

The race drew 27,000 runners and has been run since 1897 on Patriots Day, the third Monday in April, which commemorates Lexington and Concord, the two battles in Massachusetts that started the American Revolution.

The year, the race coincided with the filing deadline for federal taxes. Security experts said the FBI would undoubtedly look into the possible significance of the date as they tried to find the bomber and the motive.

NBC News' Pete Williams and Jonathan Dienst contributed to this report.

Watch the raw video of the explosion