(CNN) -- The FBI is taking the lead in investigating the double bombing that caused death and destruction near the finish line of the Boston Marathon on Monday, authorities said.
A range of different agencies are involved in the hunt to find out who carried out the lethal attack and why, but officials are cautioning that very little is known at this point.
"This will be a combined federal, state and local effort," Richard DesLauriers, the special agent in charge of the FBI's Boston Division, said at a news briefing by law enforcement officials on Monday evening.
Describing it as a "criminal investigation" that is also "a potential terrorist investigation," DesLauriers said the FBI was declaring federal jurisdiction over the matter through the Boston Joint Terrorism Task Force.
It wasn't clear Monday whether the origin of the bombings was domestic or foreign, according to a federal law enforcement official with knowledge of the investigation. And a state government official said there had been no credible threats ahead of the race.
"People shouldn't jump to conclusions before we have all the facts," President Barack Obama said. "But make no mistake, we will get to the bottom of this, and we will find out who did this."
Obama said that "any responsible individuals, any responsible groups, will feel the full weight of justice."
The FBI and the Department of Homeland Security "are mobilizing the appropriate resources to respond," according to the president.
A law enforcement official in Boston said investigators "have a number of active leads and some good early progress in the forensics analysis" but are yet to identify any suspects.
"The situation remains fluid, and it remains too early to establish the cause and motivation," the FBI's Boston Division said in a statement asking people to call in with any information, images or details related to the explosions.
"No piece of information or detail is too small," it said.
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 showed no C-4 or other high-grade explosive material, suggesting the packages used in the attack were crude devices.
U.S. Rep. Bill Keating of Massachusetts said authorities had discovered two other explosive devices that hadn't gone off.
One unexploded device was found at a hotel on Boylston Street near the bomb site, and another unexploded device was found at an undisclosed location, according to Keating, a Democrat and member of the House Homeland Security committee.
He called the bombings a "sophisticated, coordinated, planned attack."
The unexploded devices that were recovered could provide a treasure trove of information such as fingerprints and indications of the bomb maker's design, a federal law enforcement official who now works in the intelligence community said.
And from the bombs that did go off, investigators would be looking for fragments and anything indicating the "signature" of the bomb makers, the official said.
In addition to scrutinizing images of surveillance cameras in the area, the FBI was most likely issuing subpoenas for records from cell towers in the area to isolate and trace calls from around Copley Square at the time of the blasts, according to the official.
As authorities searched the scene, numerous suspicious packages were found, possibly because people fled the area, leaving items behind. Investigators were checking those objects.
Bomb-sniffing dogs were working the area of the bombings and nearby streets, checking every single item on curbs or near the street -- including "every construction cone, every Port-a-Jon" -- to make sure there were no explosive devices left, CNN affiliate WHDH in Boston reported.
Law enforcement officials were also chasing possible leads.
Investigators warned police officers to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with the Marathon bombs, according to a law enforcement advisory obtained by CNN. The man was seen with a black backpack and sweatshirt and was trying to get into a restricted area about five minutes before the first explosion, the lookout notice states.
A Saudi citizen with a leg wound was under guard at a Boston hospital in connection with the bombings, but investigators cannot say he is involved at this time and he is not in custody, a law enforcement official said Monday evening.
After initial suggestions that a third blast Monday, which took place at the John F. Kennedy Presidential Library, could be related to the marathon bombings, police said that that incident was connected to a mechanical problem.
The library said all staff and visitors were safe.
CNN's John King, Matt Smith, Steve Almasy, Josh Levs and Monte Plott contributed to this report.
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