Images from an investigation into the deadly attack at the Boston Marathon have provided more details on the type of bombs used.
Two explosions near the finish line killed three people and wounded at least 175 on Monday.
Images from a joint Homeland Security and FBI bulletin show the remains of a dark-coloured nylon backpack and remnants of a metal container that appears to be a pressure cooker.
Wires, a battery and what appears to be a small circuit board used to detonate the explosives have also been identified.
The FBI said while no-one had claimed responsibility, it has received more than 2000 tips about the bombings.
President Barack Obama will visit Boston later this week. He confirmed that the FBI was treating the bombing as an act of terror and said it was still unclear whether the perpetrators were foreign or domestic, or even whether the bombs were planted by what he called a malevolent individual.
Mr Obama urged Americans to be vigilant about suspicious activity.
An American counter-terrorism expert said the bombing had hints of a right-wing attack rather than al Qaeda-inspired extremism.
Richard Barrett said it was too early to say who caused the explosions but the timing of the attack on Patriots' Day and the relatively small size of the devices suggested the work of a domestic extremist.
The FBI said no further threats have been identified in Boston. Police said a fire at the John F Kennedy library in the city, which they describe as caused by an incendiary device, may not have been linked to the marathon attacks.
Homeland Security Secretary Janet Napolitano said there was no indication the bomb blasts were part of a broader plot.
The Boston police commissioner is describing the 12 block crime scene as the most complex his department has ever seen.
Tributes held
Residents have paid tearful tributes to those killed in the attacks, including eight-year-old Martin Richard, Krystle Campbell, 29, who was cheering on her boyfriend in the marathon, and a Chinese citizen, whose identity is not being made public at the request of their family.
More than 1000 people held a candle-lit vigil in a park near the suburban home of Martin Richard, whose mother and sister were badly injured trying to get away from the second explosion.
Other ceremonies were held in the city for the dead and injured. The mother of Krystle Campbell, Patty, said the family was heartbroken.
Surgeons at Massachusetts General Hospital worked through the night to treat the injured, many of whom have multiple injuries.
Eight are in a severe condition on Tuesday, with four having undergone major surgery, mostly amputations of lower limbs.
Doctors believed the devices may have been packed with ball bearings or nails to make them even deadlier because many of the injured had small metallic fragments like pellets or nails in their bodies.
Runners 'will return'
Runners are leaving the city but many of them are vowing to return.
Molly Graeber came from Chicago to run the marathon.
"I don't think that what happened yesterday should necessarily define the event. And I think people will come back and it will be better in future years and run in honour of those who were affected this year."
The ABC reports that many of the departing athletes wore their marathon jackets and medals which were awarded to them even if they hadn't been able to finish before the race was abandoned.
