The officials told ABC News the remains show a medium-sized pressure cooker, packed with wires, a circuit board, nails and ball bearings. The FBI says the other explosive was in a metal container, but there wasn't enough evidence to indicate that it was a pressure cooker.
It was not known what was used to set off the two explosives that killed three people Monday and injured more than 170 others.
Earlier today doctors caring for the 170-plus victims of the deadly bombing reported they have been pulling nails or nail-like objects from those struck by the twin explosions -- likely shrapnel from inside the bombs.
"Nails or sharp objects," Dr. George Velmahos, chief of trauma at Massachusetts General Hospital, said at a press conference today. "Can't say for certainty, but that's what they look like. [They're] numerous... 10, 20, 30, 40 in their bodies, maybe more."
A spokesperson for Brigham and Women's Hospital, another medical center caring for the wounded, reported similar injuries, apparently caused by carpenter nails and small ball bearings.
FBI confirms among items partially recovered are pieces of black nylon, which could be from a backpack and what appear to be fragments of BBs and nails possibly contained in a pressure cooker device. All were taken to an FBI lab for analysis.
A source with direct knowledge of the investigation tells ABC News that the bomb appears to have been placed in a black or dark colored duffel or nylon bag that was placed on the ground at one of the two sites. The official said that this may explain why there was so much damage to the lower extremities of the victims.
The public has been asked particularly for photos or video of anyone carrying an unusually large heavy black bag.
"Someone knows who did this," FBI's Rick DeLauriers said.
So far there is no definitive information on any suspects, and no claims of responsibility have been made.
ABC News is reporting the latest numbers from Boston's medical community is that as of Tuesday evening, 70 patients remain in six hospitals. Of those patients, 22 are listed in critical condition.
At the White House, meanwhile, President Barack Obama said that the bombings were an act of terrorism but that investigators do not know if they were carried out by an international organization, domestic group or a "malevolent individual."
He added: "The American people refuse to be terrorized."
President Obama is planning to travel to Boston on Thursday to attend a memorial service to be held at 11am ET, as announced by Massachusetts Governor Deval Patrick.
Across the U.S., from Washington to Los Angeles, police tightened security, monitoring landmarks, government buildings, transit hubs and sporting events. Security was especially tight in Boston, with bomb-sniffing dogs checking Amtrak passengers' luggage at South Station and transit police patrolling with rifles.
Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass. The dead included an 8-year-old boy.
Federal investigators said no one had claimed responsibility for the bombings, which took place on one of the city's biggest civic holidays, Patriots Day.
"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," said Richard DesLauriers, FBI agent in charge in Boston.
He said investigators had received "voluminous tips" and were interviewing witnesses and analyzing the crime scene.
Gov. Deval Patrick said that contrary to earlier reports, no unexploded bombs were found. He said the only explosives were the ones that went off.
Late Monday a tip about possible explosives led federal agents to search an apartment on the fifth floor of a building in the Boston suburb of Revere, but agents later told residents there was nothing to worry about. The quick and overwhelming law enforcement response underscored the urgency of the FBI's effort to track and stop the people responsible.
Monday FBI agents also went to a local hospital to question a 20-year-old Saudi college student who was injured in the blast, but authorities stressed that he is not considered a suspect.
Per the Saudi Embassy in Washington DC, US authorities "have confirmed that there is no Saudi who is a suspect in the bombing and that the person being questioned is a witness and not a suspect. That has been the case since yesterday. He has been fully cooperating with the authorities."
At a news conference, police and federal agents repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people might not think are significant.
"There has to be hundreds, if not thousands, of photos and videos" that might help investigators, state police Col. Timothy Alben said.
Boston Police Commissioner Edward Davis said investigators also gathered a large number of surveillance tapes from businesses in the area and intend to go through the video frame by frame.
"This is probably one of the most photographed areas in the country yesterday," he said.
The fiery explosions took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending columns of smoke rising over the street.
Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the explosions.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated."
At least 17 people were critically injured, police said. At least eight children were being treated at hospitals. In addition to losing limbs, victims suffered broken bones, shrapnel wounds and ruptured eardrums.
Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it -- similar in the appearance to BBs."
At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
Eight-year-old Martin Richard was among the dead, said U.S. Rep. Stephen Lynch, a family friend. The boy's mother, Denise, and 6-year-old sister, Jane, were badly injured. His brother and father were also watching the race but were not hurt.
A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walk.
Neighbor Betty Delorey said Martin loved to climb the neighborhood trees, and hop the fence outside his home.
Tim Davey of Richmond, Va., was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving. "They just started bringing people in with no limbs," he said.
"Most everybody was conscious," Lisa Davey said. "They were very dazed."
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. Most of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course.
The attack may have been timed for maximum bloodshed: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.
Davis, the police commissioner, said authorities had received "no specific intelligence that anything was going to happen" at the race. On Tuesday, he said that two security sweeps of the route had been conducted before the marathon.
The race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
Richard Barrett, the former U.N. coordinator for an al-Qaida and Taliban monitoring team who has also worked for British intelligence, said the relatively small size of the devices in Boston and the timing of the blasts suggest a domestic attack rather than an al-Qaida-inspired one.
"This happened on Patriots Day -- it is also the day Americans are supposed to have their taxes in -- and Boston is quite a symbolic city," said Barrett, now senior director at the Qatar International Academy for Security Studies.
The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, on Tuesday denied any role in the bombings.
A woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."
After a minute or so without another explosion, Wall said, she and her family headed to a Starbucks and out the back door through an alley. Around them, the windows of the bars and restaurants were blown out.
She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood trickling down his head. Another person was on the ground covered in blood and not moving.
"My ears are zinging. Their ears are zinging," Wall said. "It was so forceful. It knocked us to the ground."
ABC News and the Associated Press contributed to this report.
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