The suspects are described as two white men in their 20s, and the FBI is calling them armed and dangerous.
The FBI also released images and footage of the suspects taken near the race finish line. The first suspect is seen wearing a dark cap, a backpack and sunglasses, making it difficult to see his face. The second suspect is seen wearing a white backwards cap, giving viewers a clear shot of his face. He also carried a light-colored backpack.
The pair is seen in the footage walking together through the marathon crowd in the direction of the finish line. The second suspect set down a backpack at the site of the second explosion, right outside a restaurant, according to FBI Special Agent Richard Deslauriers.
The FBI urged the public to report tips to BostonMarathonTips.FBI.gov or at (800) CALL-FBI.
ABC News has also learned that a still of one of the suspects using a cellphone has been located. It is believed the suspect was using the cellphone to trigger one of the bombs. Cellphone records will be investigated for the entire Boston area at the time of the bombings.
Law enforcement officials have photos of the potential suspects from surveillance footage from a Lord & Taylor along the race route. Federal law enforcement sources say neither suspect has been positively identified by name. They believe each suspect placed a bomb at the site.
Investigators are sorting through the hundreds of videos and pictures from the area surrounding the twin bomb blasts that erupted near the marathon finish line Monday, killing three people and injuring more than 170 others. No group, foreign or domestic, has claimed responsibility for the attack.
As of Thursday morning, at least 14 bombing victims, including three children, remained in critical condition. Dozens of victims have been released from hospitals, and officials at three hospitals that treated some of the most seriously injured said they expected all their remaining patients to survive.
Richard Clarke, a former White House counter-terrorism advisor and current ABC News consultant, said a special FBI team is likely using facial recognition software to try and match any suspicious individuals to information the government may have on file.
Federal agents suspect components of a toy DuraTrax radio-controlled car were used to construct the bombs that exploded at the Boston Marathon Monday, according to four local hobby store owners and managers interviewed by the agents.
Store owners in Massachusetts and across the border in New Hampshire told ABC News federal agents questioned them about sales of a 1.25 volt Tenergy battery used in remote and radio-controlled cars, the same kind shown in evidence photos from the scene of Monday's bombing.
FBI agents also made inquires at California-based Tenergy, which makes the battery packs specifically for use in DuraTrax toy cars, a company official said.
President Obama attends service for Boston bombing victims
An interfaith prayer service was held Thursday morning to honor the victims of the Boston Marathon bombings. President Barack Obama spoke at the emotional event at the Cathedral of the Holy Cross in Boston, along with Boston Mayor Thomas Menino and Massachusetts Gov. Deval Patrick. The crowd heard music from cellist Yo-Yo Ma and the Boston Children's Chorus.
"For millions of us, what happened on Monday is personal," the president told the crowd.
Mr. Obama paid tribute to the city shaken by what he has called an act of terror, saying, "There is a piece of Boston in me," and remembered the victims lost: 29-year-old Krystle Campbell of Medford, Mass., Boston University graduate student Lingzi Lu and 8-year-old Martin Richard.
"We're left with two enduring images of this little boy, forever smiling for his beloved Bruins and forever expressing a wish he made on a blue poster board: 'No more hurting people. Peace,'" Mr. Obama said.
"Every one of us stands with you," the president said. "You will run again!"
Hundreds lined up to attend the first-come-first-serve event. Bostonians said the service was inspiring and provided some sense of closure.
"It's the people of Boston who have been put down and they're not going to stay down. Boston is a very strong city," Boston resident Pauline Dicesare said.
Shortly after the event, the president met with a running group at a high school, and then spent time at local hospitals, meeting with some of the victims and thanking emergency workers. First lady Michelle Obama spent time at a children's hospital.
The Associated Press and ABC News contributed to this report.