Twin explosions have caused three deaths and many more injuries at the Boston marathon in the US. In a statement President Barack Obama said the perpetrators will feel the "full weight of justice."
The explosions struck in Boston roughly three hours after the winning runners had crossed the finish line. The blasts hit the north side of Boylston Street, near the Fairmont Copley Plaza Hotel and the bridge that marks the finish line of the marathon course.
"These were powerful devices that resulted in serious injury," Davis told reporters. Massachusetts Governor Deval Patrick added that more than 100 people were injured, some gravely.
The Boston Globe reported there were at least 144 people wounded, with injuries ranging from cuts and bruises to amputations. It said an eight-year-old child was among the dead.
According to Richard DesLauriers, the special agent in charge of the FBI's Boston office, the bureau had taken the lead in investigations. He said officials were treating it as a criminal investigation "that is a potential terrorism investigation."
DesLauriers declined to comment on reports that additional explosive devices had been located around the city.
There was also no immediate information on where the explosive devices might have come from and police have denied reports that a suspect was being held in custody.
Obama promises justice
At the White House US President Barack Obama said it was not yet clear who was behind the blasts.
"We still don't know who did this, or why, and people should not jump to conclusions before we have all the facts, but make no mistake, we will get to the bottom of this," Obama said in statement broadcast live on national television.
Like police officials in Boston, the president was keen to reiterate that investigations into the explosions remained at an early stage and made no reference to the attack as a terrorist incident.
An unnamed White House official later released a statement, however, saying it would be treated by the authorities as 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.
Security stepped up
According to the Boston Police Commissioner the explosions occurred 50 to 100 yards apart at around 3 p.m. local time (1900 UTC).
Roughly 90 minutes later a fire erupted at the John F. Kennedy Library in Boston. Media outlets initially labeled the incident as an explosion, possibly linked to the marathon blasts, but police have now said it no longer appears to be related.
"The preliminary investigation indicates JFK incident may not have been an explosion. It may have been a fire," Commissioner Davis tweeted.
In a separate tweet Davis said the Boston Police Department was on "high alert." He has also urged citizens to remain vigilant.
Meanwhile security was stepped up in New York, Washington, Los Angeles and San Francisco.
The annual Boston marathon is traditionally held on Patriots' Day, a local state holiday commemorating the American Revolutionary War battles of Lexington and Concord in April of 1775.
ccp, msh/av (AFP, AP, dpa, Reuters)
Note: The Boston mayor's office has set up a telephone helpline for people concerned for friends or family who might have been caught up in the explosions. The number is: 001-617-635-4500.