Washington: Investigators probing the twin Boston bombings that left three people dead have a number of active leads and have made "some good early progress in the forensics analysis", a law enforcement official said.
The Federal Bureau of Investigation (FBI) is taking the lead in investigating the double bombing that left at least 144 injured at the Boston Marathon finish line.
A range of different agencies are involved in the hunt to find out who carried out the terrorist attack Monday afternoon in one of the oldest cities in the US and an international centre of higher education and medicine.
Harvard University, the nation`s oldest, is located across the Charles river in Cambridge. Its business and medical schools are in Boston, the capital city of Massachusetts State.
"This will be a combined federal, state and local effort," Richard DesLauriers, the special agent in charge of the FBI`s Boston office, told reporters Monday evening, according to CNN.
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.
A law enforcement official in Boston cited by CNN said investigators "have a number of active leads and some good early progress in the forensics analysis" but are yet to identify any suspects.
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.
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 Democrat Bill Keating, a member of the House Homeland Security committee.
The unexploded devices that were recovered could provide a treasure trove of information such as fingerprints and indications of the bomb maker`s design, CNN said citing an intelligence official.
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 was quoted as saying.
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.
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 bombing, CNN said citing a law enforcement advisory.
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.
After initial suggestions that a third blast Monday, which took place at the John F. Kennedy Presidential Library, was related to the Boston Marathon bombings, police said that incident was believed to be fire-related.
After the blast at JFK Library, the JFK School of Government at Harvard was evacuated as authorities feared similar sounding institutions might be targeted.
"We will turn every rock over to find the people who were responsible for this," Boston Police Commissioner Edward F. Davis said, according to Boston Globe.
No suspects have been arrested, Davis said, though police were questioning many people.
Davis said the attacks had occurred without warning. "There was no specific intelligence" suggesting an attack would take place, he said.