The FBI will lead the investigation of the blasts that killed three and injured more than 130 near the finish line of the Boston Marathon, officials said.
Richard DesLauriers, special agent in charge of the FBI's Boston Division, called the matter a "criminal investigation" that also was a "potential terrorist investigation," CNN reported Tuesday.
Two bomb blasts, 12 seconds apart, shook the finish line of the 117th running of the Boston Marathon Monday, killing at least three people, including an 8-year-old boy, wounding more than 130, and leaving the sidewalks along the race court bloodied. Some media reports indicate as many as 144 people were injured.
DesLauriers said the FBI declared federal jurisdiction through the Boston Joint Terrorism Task Force.
"This will be a combined federal, state and local effort," he said Monday evening.
The FBI has asked people to contact the agency with images, information or details concerning the two explosions that occurred about 4 hours and 9 minutes into the marathon Monday near the finish line.
"No piece of information or detail is too small," the FBI's Boston Division said in a statement.
Still unclear was whether the bombings were conducted by a domestic or foreign person or persons, a federal law enforcement official knowledgeable of the investigation told CNN. A state government official said no credible threats were discovered ahead of the race.
"People shouldn't jump to conclusions before we have all the facts," President Barack Obama said in addressing the nation Monday evening. "But make no mistake, we will get to the bottom of this, and we will find out who did this."
A White House official said the bombing was "clearly an act of terror."
"Any event with multiple explosive devices, as this appears to be, is clearly an act of terror, and will be approached as an act of terror," the official said.
A law enforcement official in Boston said investigators "have a number of active leads and some good early progress in the forensics analysis" but have yet to identify any suspects.
The explosions blew out windows, knocked down runners and spectators, shot smoke into the sky and left victims falling over each other on Patriots Day, a civic holiday in Massachusetts, when schools and offices are closed to commemorate the anniversary of the 1775 Battles of Lexington and Concord, the first battles of the American Revolutionary War.
About 30 people were transferred to hospitals under a Code Red, meaning life threatening injuries, The Boston Globe reported.
Flags were lowered to half-staff around the nation.
Boston Police Commissioner Ed Davis said many people were being questioned.
A federal law enforcement official told CNN that both bombs were small, and initial tests indicated they lacked high-grade explosive material.
U.S. Rep. Bill Keating, D-Mass., said authorities had discovered two unexploded devices.
As authorities searched the scene, numerous suspicious packages were found, CNN said. Investigators were checking those objects. Bomb-sniffing dogs were working the area of the bombings and nearby streets, WHDH-TV, Boston reported.
Investigators alerted police officers to be on the lookout for a "darker-skinned or black male," possibly with a foreign accent, a law enforcement advisory obtained by CNN said. The man was seen dressed in a hooded sweatshirt and carrying a black backpack and was trying to enter a restricted area about five minutes before the first blast, the notice said.
A Saudi citizen with a leg wound was being guarded at a Boston hospital and was cooperating with the FBI, The Boston Globe reported. Investigators said the man told agents he was not involved in the explosions and ran because he was frightened, the Globe said. Investigators did not characterize the man as a suspect.
The Globe also reported law enforcement officials Monday stormed an apartment building and conducted a search as part of the investigation.
Police locked down a 15-block area around the scene.
Massachusetts Gov. Deval Patrick, who gave updates Monday afternoon and evening, said the blast area was expected to be closed Tuesday.
He said Boston "will not be business as usual," in the wake of the terror attack and police presence will be heavy in the city.
After initial suggestions a third fire at the John F. Kennedy Presidential Library could be related to the marathon bombings, police later ruled the library incident was tied to a mechanical problem.
About three-quarters of the 23,000 runners had crossed the finish line when the bomb, which witnesses said appeared to have been in a garbage can, exploded on Boylston Street, just off Copley Square in the heart of Boston.
The second bomb exploded on the other side of the street, several hundred feet away.
In other major cities, including New York and Washington, officials stepped up security at important locations. In Washington, the Secret Service cordoned off Pennsylvania Avenue near the White House out of what one official described as "an abundance of caution."
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