NEW YORK CITY — A pair of powerful bombs exploded just steps from the finish line of the Boston Marathon Monday afternoon, killing at least three and injuring more than 170 runners and spectators, according to authorities.
Police also purposely detonated another suspicious package they found along the marathon route in the chaotic aftermath as emergency responders treated bloodied victims — including at least seven children — and confused bystanders fled for safety.
But by Tuesday, Gov. Deval Patrick said that no additional bombs had been found.
President Barack Obama addressed the nation from the White House Monday evening, vowing to get answers about the bombings, which the FBI said they were investigating as potential terrorism. New York was on high alert, with beefed up NYPD presence at subways and major landmarks. More than 700 New Yorkers competed in the event, according to the official registry.
"We don't yet have all the answers, though we do know that multiple people have been wounded, some gravely, in explosions at the Boston Marathon," Obama said.
"Any responsible individuals, any irresponsible group, will feel the full weight of justice.”
The president did not refer to the blasts as a terrorist attack, but a federal law enforcement source said the FBI was treating the seemingly coordinated explosions as an act of terror.
FBI agent Richard DesLauriers said at a press conference Monday night that his agency was taking the lead in the probe.
“It is a criminal investigation that is a potential terrorist investigation,” he said.
"This will be a world-wide investigation," he added Tuesday. "We will go to the ends of the earth" to capture those responsible, he said.
The explosions drew a swift response from the NYPD, which mobilized anti-terror units, bomb squads and additional uniformed presence to key New York City landmarks and buildings.
"We've been stepping up security at hotels and other prominent locations in the city through deployment of the NYPD's critical response vehicles (CRVs) until more about the explosions in Boston is learned," the department's spokesman, Paul Browne, said in an email.
"We're ramping up everything," another NYPD police official told DNAinfo.com New York.
The bombs exploded within seconds of each other at 2:50 p.m. and about 50 to 100 yards apart. They occurred four hours after the start of the annual footrace as thousands of runners, including many New Yorkers, were still drifting over the line.
Video shows one of the blasts going off inside a Marathon Sports store on Boylston Street, blowing out glass windows and sending shrapnel and a huge mushroom cloud of smoke onto a sidewalk packed with cheering onlookers.
"It was like 9/11," said Michael Arnstein, 36, of New York City, who completed the race with his wife 5 minutes before the blasts. "When that second bomb went off, everybody panicked."
Arnstein told DNAinfo.com New York that he was 900 feet from the explosions and described how "the first one went off and everything shook."
"There was a big cloud of smoke and then a few seconds later a second thing went off and that was confirmation," he said.
Boston Police Commissioner Ed Davis said the explosions killed three — including an 8-year-old boy — and injured more than 170 people. Six children were taken to Boston Children's Hospital. Their conditions ranged from serious to good. It was unclear if New Yorkers were among the victims.
Davis said police were questioning several people, but no one was under arrest and there was no suspect so far.
He described the area of the attack Tuesday as "the most complex crime scene in the history" of the city's department.
A federal law enforcement source said that in the aftermath, a marathon viewer tackled a fleeing man he thought looked suspicious. The source said the tackled man was being questioned but had been cooperative.
Images of the carnage showed runners with gashes on their heads and blood on the asphalt below.
Runners were treated in the tents beside the race route and others were whisked away by ambulance.
Officials said people could call 617-635-4500 to find out if a loved one had been injured in the blasts. Authorities said witnesses could also call 1-800-494-TIPS if they have any information.
Spectators and participants scattered after the blasts — with some leaving behind bags and parcels. Davis said police were treating the left-behind belongings as suspicious devices, but none so far contained explosives.
The police commissioner encouraged people in Boston to head to their homes.
"People should be calm. They should understand that this is an ongoing event and we need all the information available to us," he said.
Late Monday night Massachusetts Gov. Deval Patrick said the National Guard had cordoned off the area around the blasts, calling it an active crime scene.
"The city of Boston is open and will be open tomorrow, but it will not be business as usual," he said. "There'll be a heightened law enforcement presence consistent with the severity and seriousness of the ongoing investigation."
At an earlier press conference, Davis said they were investigating an incident at JFK library that occurred at 4:30 p.m. He later tweeted that a preliminary probe showed it was a fire. A library spokesman said there were no injuries.
More than 700 people from New York competed in the Boston Marathon this year. About 532 competitors were from Manhattan, 145 from Brooklyn, 10 from Queens, 17 from the Bronx and seven from Staten Island, according to marathon organizers.
Mayor Michael Bloomberg asked that New Yorkers keep the victims and their families in their prayers. He added that 1,000 members of the NYPD's counter-terrorism unit had been activated.
“I have spoken with Police Commissioner Ray Kelly, and the NYPD has stepped up security at strategic locations and critical infrastructure, including our subways. Some of the security steps we are taking may be noticeable, including deployment of Critical Response Vehicles and additional police personnel, and others will not be," he said.
"We have 1,000 members of the NYPD assigned to counter-terrorism duties, and they — along with the entire NYPD and the investments we have made in counter-terrorism infrastructure — are being fully mobilized to protect our city.”
The New York City MTA said its police force would also step up patrols and bag inspections at transit hubs.
"The MTA is increasing patrols, coverage and bag inspections. All security personnel will remind all employees to be vigilant," MTA spokesman Kevin Ortiz said. "The increased coverage will continue until we fully understand the cause of the explosions in Boston. NYPD is also on alert and indicated they will give additional attention to the subway system."
Gov. Andrew Cuomo noted that on Sunday the New York National Guard had sent three vehicles and six soldiers to support the Boston Marathon. He added that New York agencies were at a heightened state of vigilance.
"Our thoughts and prayers are with the people of Boston and Massachusetts as reports come in on the horrific tragedy at the Boston Marathon," he said. "I have directed state agencies, including the Division of Homeland Security and Emergency Services, State Police, the MTA and the Port Authority, to be on a heightened state of alert as we learn more about this incident."
During the New York City Marathon, the NYPD searches all runners and participants in the staging area and finish line, according to a police source. In the 2011 race, cops confiscated 20 to 30 backpacks, the source said.
Mary Wittenberg, the head of the New York Road Runners, which organizes the New York City Marathon, called the bombing "a tragic day for all of us in the running community."
"The safety and security of all New York Road Runners’ races is and will always be our top priority," Wittenberg said. "We will continue to work hand in hand with the city of New York and the NYPD as we plan for upcoming events.”
An explosives expert told DNAinfo.com New York that the explosions were likely pipe bombs made of gunpowder because people at the scene reported the smell of sulfur. Pipe bombs are stuffed with the powder and ball bearings and can be detonated with a timer or cell phone, the expert said.