FBI Agent in Charge Richard DesLauriers said it was "possible" the homemade explosive devices were cookers loaded with nails and other metal objects. He said they could have been transported in "a heavy, dark colored bag" or backpack, the Boston Herald reported.
The agent said the FBI had been flooded with 2,000 tips.
DesLauriers said forensic experts were "rebuilding" the bombs and the FBI had found pieces of "black nylon" from a backpack or bag at both blast sites, the Herald said.
The agent wouldn't comment on reports a circuit board had been found among the blast debris.
The Boston Globe reported an official it described as briefed on the situation said investigators recovered a circuit board near one of the bomb sites and they believe it was used to detonate the bombs. That source also said the investigators had determined the bombs were made from two 6-liter pressure cookers filled with nails, ball bearings and other metal, the Globe said.
The dead included 8-year-old Martin Richard of Dorchester, Mass., whose mother and sister were seriously injured; Krystle Campbell, 29, of Medford, Mass., who was cheering on her boyfriend at the finish line; and a Chinese national who was a student at Boston University, the Globe said.
Neither the Chinese Consulate in New York nor the university identified the BU student, the Globe said.
No arrests had been made in the attack, which also left 176 people wounded and which President Barack Obama called "evil" and an "act of terrorism."
A number of the injured were hospitalized in critical condition at Boston-area hospitals.
U.S. House Homeland Security Committee Chairman Michael McCaul, R-Texas, told The New York Times investigators said the devices were similar to those used against U.S. troops in Afghanistan and Iraq, instructions for which are available on the Internet. The Times quoted a law enforcement official as saying the bombs likely used black powder detonated by ordinary kitchen-type timers.
A federal law enforcement official told CNN both bombs were small, and initial tests indicated they lacked high-grade explosive material.
Boston police and firefighters unions began organizing a Boston First Responders Fund to offer a $50,000 reward for information leading to the arrest of those responsible, the Globe said.
"I want to cry ... but we must not give in to fear," Boston Police Patrolmen's Association President Tom Nee said.
"We will find the person or persons responsible for disrupting the way of life here in Boston."
Obama, in remarks from the White House, hailed the actions of those who aided the wounded after the explosions erupted near the finish line Monday.
"The American people refuse to be terrorized," said Obama, citing the actions of those who exhibited "heroism and kindness and generosity and love, exhausted runners who kept running to the nearest hospital to give blood and those who stayed to tend to the wounded, some tearing off their own clothes to make tourniquets, the first responders who ran into the chaos to save lives, the men and women who are still treating the wounded at some of the best hospitals in the world, and the medical students who hurried to help, saying, 'When we heard, we all came in,' the priests who opened their churches and ministered to the hurt and the fearful, and the good people of Boston who opened their homes to the victims of this attack and those shaken by it.
"So if you want to know who we are, what America is, how we respond to evil, that's it: selflessly, compassionately, unafraid.
"This was a heinous and cowardly act. And given what we now know about what took place, the FBI is investigating it as an act of terrorism. Any time bombs are used to target innocent civilians, it is an act of terror," Obama said.
Obama said it was unclear who planted the bombs, saying investigators have yet to determine whether it was a terrorist organization, "domestic or foreign," or a lone "malevolent" individual was involved.
"Clearly we are at the beginning of our investigation," he said. "It will take time to follow every lead and determine what happened, but we will find out. We will find whoever harmed our citizens, and we will bring them to justice."
Suspicious packages found and detonated by bomb squads as a precaution after the explosions were not explosive devices, officials said.
Investigators say the only bombs were the two that exploded at the marathon. A doctor treating some of the wounded at Massachusetts General Hospital said patients had nails, or nail-like sharp objects, and "pellets" in their bodies.
In opening remarks before the House Appropriations Defense subcommittee, Defense Secretary Chuck Hagel said the bombings were a "cruel act of terror."
Police locked down a 15-block area around the scene and the blast area remained closed Tuesday.
Massachusetts Gov. Deval Patrick said Boston "will not be business as usual" in the wake of the terror attack and police presence would be heavy.