The FBI says there have so far been no claims of responsibility for the Boston Marathon bombings.
Investigators also now say they believe the blasts may have been caused by metal "pressure-cooker" devices that were carried in dark-colored bags or backpacks.
The double-bombing April 15 near the marathon's finish line killed three people and injured more than 170.
Officials said those killed in the bombings include an eight-year-old boy, a 29-year-old woman – both Americans – and a female Chinese graduate student at Boston University.
FBI Special Agent Rick DesLauriers said investigators have recovered items from both blast sites, including pieces of black nylon that may be from a bag that may have hidden the bombs.
He said the bombs may have been built using a “pressure-cooker device,” and that fragments of steel pellets and nails from the bombs had been recovered.
News broadcasters later on April 16 showed crime-scene photographs of what appeared to be twisted pieces of a metal container and remnants of a torn black bag.
An Al-Qaeda branch in Yemen published an online article in 2010 describing techniques for producing pressure-cooker bombs, and such bombs have been used in Pakistan, Afghanistan, India, and Nepal.
But instructions for building pressure-cooker bombs are available on the Internet and could be made relatively easily and cheaply by anyone, including a domestic terrorist.
President Barack Obama said the bombings are being investigated as an act of terror, but said officials don't know "whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual."
"This was a heinous and cowardly act and given what we now know what took place, the FBI is investigating it as an act of terrorism," Obama said. "Any time bombs are used to target innocent civilians it is an act of terror."
The White House said Obama is expected to visit Boston on April 18 to speak at an interfaith service dedicated to the victims of the bombings.
U.S. Homeland Security Secretary Janet Napolitano said there was no indication that the bomb blasts were part of a broader plot targeting America.
The Boston bombings mark the deadliest attack targeting civilians in the United States since the September 11, 2001 hijacked airliner attacks claimed by the Al-Qaeda network.
In his remarks, FBI Special Agent DesLauriers said the search for suspects and motives remains “wide open” and called on members of the public to come forward with any relevant information.
Saudi Arabia, the Gulf Cooperation Council, and the Organization of Islamic Cooperation have meanwhile joined global condemnations of the bombings.
King Abdullah of Saudi Arabia, in a message of condolence to the American people, said the perpetrators were “criminals who represent only themselves and have neither religion nor faith."
With reports from Reuters, AP, and AFP