President Obama vowed on Monday that those behind the bombing which killed at least three people at the Boston Marathon "will feel the full weight of justice."
"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts, but make no mistake, we will get to the bottom of this, and we will find out who did this," Obama said, offering condolences to victims and their families and praising first responders.
"We will find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice," Obama added.
The president was briefed within minutes of the explosions that killed at least three people and injured more than 140 more near the finish line of the marathon, as the federal government boosted security measures and offered pledges of law enforcement support to the investigation.
Two bombs exploded in the crowded streets near the finish line of the Boston Marathon on Monday , killing at least three people and injuring more than 140 in a bloody scene of shattered glass and severed limbs that raised alarms that terrorists might have struck again in the U.S.
Reports emerged that among the dead was an 8-year-old child.
A Saudi national, believed to be a university student in Boston, was reportedly questioned in the hospital receiving casualties.
“The man was tackled and held by a ¬bystander after he was seen running from near the scene of the explosion,” the Boston Globe reported, citing a law enforce¬ment source who spoke with another source involved in the FBI’s investigation.
The Saudi man “is cooperating with the FBI and told agents that he was not involved in the explosions, and that he ran only because he was frightened,” the report added.
Investigators did not state the man is being treated as a suspect.
The twin blasts sent authorities out on the course to carry off the injured while the stragglers in the 26.2-mile (40 kilometer) trek were rerouted away from the smoking site of the blasts.
Al Arabiya's correspondent said that subway trains were stopped, as Boston police called on the residents of the city to stay in their homes.
Also, off-duty police officers were called in to help with the situation. Meanwhile, according to CNN, a hotel located near the finish line was evacuated following the blast.
A third explosion was heard about an hour following the first two after authorities warned spectators to expect a loud noise from a water cannon that police apparently were using to destroy one of the devices.
A law enforcement official said mobile phone service has been shut down in the Boston area to prevent any potential remote detonations of explosives.
Boston Police Commissioner Edward F. Davis said that there was also an explosion at the John F. Kennedy presidential library, though there were no known injuries. He urged people to stay indoors and not congregate in large groups, adding there were no suspects in custody but there were "many people" being questioned.
The Boston police commissioner also said that investigators have not linked the fire at JFK Library to the marathon blasts, adding that authorities have not identified what caused the explosives that erupted at the finish line of the Boston Marathon.
Competitors and race organizers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.
“There are a lot of people down,” said one man, whose bib No. 17528 identified him as Frank Deruyter. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg. A Boston police officer was wheeled from the course with a leg injury that was bleeding.
About three hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another thunderous explosion could be heard a few seconds later.
Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.
“There are people who are really, really bloody,” McLean said. “They were pulling them into the medical tent.”
Cherie Falgoust was waiting for her husband, who was running the race.
“I was expecting my husband any minute,” she said. “I don’t know what this building is... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked.”