There have been no claims of responsibility for the attack, DesLauriers told reporters Tuesday evening.
More than 1,000 law enforcement officers from different agencies have been assigned to this investigation, the FBI agent said, and investigators are working around the clock.
"We will go to where the evidence and the leads take us," DesLauriers said at a media briefing earlier in the day.
Timeline of Boston Marathon bombing
"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," he said.
The Associated Press reported earlier Tuesday that a person briefed on the investigation, who was not identified, said the bombs were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings.
During the Tuesday evening news conference, DesLauriers said investigators have recovered several items, including pieces of black nylon that could be from a backpack and what "appear to be fragments of BBs and nails possibly contained in a pressure cooker device."
The FBI agent wasn't definitive and said investigators won't know precisely what components were used in the device until the lab has finished its final review.
- Boston bombing in photos
George Velmahos, the chief of the trauma department at Massachusetts General Hospital, said doctors working on patients encountered "small metallic fragments" that were more consistent with pellets, but also other pieces of metal, including some that had "spiked points that resembled nails without heads."
Dr. Tracey Dechert, of the Boston Medical Center, said she hadn't seen nails or ball bearings.
"We've been removing various things from people," she said, noting that doctors had removed particles of metal and plastic.
The near-simultaneous explosions occurred Monday near the finish line of the race — about 100 metres apart. Boston Globe photographer John Tlumacki said the bombs went off 1½ hours after the first runners crossed the finish line.
"The initial blast jolted me and dropped a runner onto the ground." That man, who suffered minor injuries, has been identified as 78-year-old Bill Iffrig of Washington state, who was running his third Boston Marathon.
Earlier Tuesday, both Boston police Commissioner Edward Davis and Gov. Deval Patrick clarified earlier reports that said up to seven unexploded bombs may have been found in the city, confirming that only two bombs were involved in the attack.
At the time, Davis said 176 people were injured and three died in the attack.
3 killed in explosions
One of the three killed was an eight-year-old boy identified in local media reports as Martin Richard of Boston's Dorchester neighbourhood.
Richard was at the finish line watching the race with his family, according to U.S. Representative Stephen Lynch, a friend of the family. Martin's mother and his siter were badly injured. His brother and father were also watching the marathon but reports said they weren't injured.
The Associated Press identified another of the victims as 29-year-old restaurant manager Krystle Campbell of Medford, Mass.
William Campbell says his daughter, who worked at a restaurant in nearby Arlington, was a "very caring, very loving person and was daddy's little girl." He says the loss devastated the family.
Campbell was at the race with a friend to see the friend's boyfriend cross the finish line, her father said. The friend was also injured in the blast, AP reported.
- 8-year-old boy, 29-year old restaurant manager killed in Boston explosions
Boston University said a graduate student at the school was the third victim. In a statement late Tuesday afternoon, the school said it was not releasing the name or any other information about the student, pending permission from the family.
The Chinese Consulate in New York acknowledged in a statement posted Tuesday night that a Chinese national had been killed.
"At the request of her family, the victim's personal information will not be disclosed," the statement said, adding that another Chinese national was in hospital after being injured in the blast.
"Our hearts go out to the families of the victims of this terrible tragedy," the statement said.
"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," U.S President Barack Obama said on Tuesday.
"We will pursue every effort to get to the bottom of what happened," he added.
State police said officers with a search warrant related to the investigation searched a fifth-floor apartment unit Monday night in the Boston suburb of Revere, but provided no further details. Investigators were seen leaving the building early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
The Pakistani Taliban have threatened to attack the U.S. in the past, but the group’s spokesman Ahsanullah Ahsan denied involvement in a telephone call with The Associated Press.
The Boston Marathon is held on Patriots Day, a civic holiday that commemorates one of the first battles of the American Revolution.
Lower-leg injuries common
Eight hospitals said they were involved in treating people. The injuries ranged from cuts and bruises to amputations. Many victims suffered lower-leg injuries and shrapnel wounds. Some suffered ruptured eardrums.
Among the badly injured spectators are two brothers, ages 31 and 33, from the Boston area, who each lost a leg from the knee down. Despite early estimates at the scene, it was not clear Tuesday evening exactly how many people lost limbs in the explosions.
Velmahos said some of the patients his hospital treated who had lost limbs expressed gratitude that they were alive because they thought they were going to die.
"It's almost a paradox to see these patients without an extremity to wake up and feel lucky – but that talks about our patients," he said Tuesday afternoon.
- Boston Globe video of marathon explosion
Investigators are examining the remains of the explosive devices, described as relatively small and crude.
The blasts shattered windows and sent dense plumes of smoke rising over the street. Emergency workers and national guardsmen on crowd control duty began climbing over and tearing down temporary fences to get to the blast site.
Tears as people fled chaos
Competitors and race organizers were crying as they fled the chaos. Bloody spectators were carried to the medical tent that had been set up to care for fatigued runners.
"They just started bringing people in with no limbs," runner Tim Davey, of Richmond, Va., told The Associated Press. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but "they saw a lot."
Mike Murphy, a former New Brunswick health minister, was there with his wife Moira at the finish line of the marathon, waiting for their son Timothy to finish. He told CBC News that after the explosions his wife helped a woman in front of her whose running gear was "melting into her skin."
Foreign Affairs officials said Tuesday there are no reports of any Canadians injured in the blasts.
According to the Boston Marathon website, 2,078 Canadians were listed as entrants among the 23,000 people in the race.
Canada in 'close consultation' with U.S. security
"We have had, of course, as you would expect, close consultation with security officials in the United States," Canada's Defence Minister Peter MacKay said on Tuesday.
"I won't speak to any specific change in the security atmosphere here, but certainly we are always vigilant in the aftermath of an incident such as this."
- Canadians describe blast 'fear' and 'shock'
Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.
"There are people who are really, really bloody," McLean said. "They were pulling them into the medical tent."
Calgary’s Kirsten-Ellen Fleming had been watching the race near the finish line and then headed into a restaurant on the corner of Boylston and Newbury streets.
“We were coming out of the restaurant and there were two big explosions and a bunch of smoke.… It was just immediate chaos and mayhem and confusion,” she said.
"Everyone was just in utter shock and looking around."
Some runners were still going and confused, she said. “Within a very short amount of time, minutes, there were just sirens everywhere. It just sounded like an absolute war zone. People were crying, on the pavement, like sitting down with their head between their knees. Everyone was on their phone, obviously calling loved ones to let them know they were OK."
The executive director of the Boston Athletic Association, which puts on the marathon, issued a statement Tuesday extending sympathies to those who were killed and injured, and thanking everyone who assisted in the aftermath of the explosions.
"What was intended to be a day of joy and celebration quickly became a day in which running a marathon was of little importance," Thomas Grilk said in the statement.
The BAA thanked all volunteers and first responders, and said it was co-operating with investigators. Grilk also said the marathon would go on next year.
"The Boston Marathon is a deeply held tradition – an integral part of the fabric and history of our community," he said "We are committed to continuing that tradition with the running of the 118th Boston Marathon in 2014."
Boston's police commissioner told reporters on Tuesday the area around the crime scene has been reduced from 15 blocks to 12, but that it may take two more days to collect all the evidence there before it’s completely re-opened to the public.
"There has to be hundreds if not thousands of photos or videos or observations that were down at that finish line yesterday," said Timothy Alben, superintendent of the Massachusetts State Police at Tuesday's briefing in Boston.
Meanwhile, security precautions are in place at the Canadian consulate in Boston, but the building is open.