This story was last updated at 3:57 p.m.
Law enforcement authorities Tuesday said they were following up on leads from the public but that no one was in custody after two bombs exploded near the finish line of the Boston Marathon a day earlier.
President Obama spoke at the White House Tuesday morning.
"It will take time to follow every lead and find out what happened," he said.
The president added that it is not yet known who carried out the attack in Boston and said, "we do not have a sense of motive yet."
The two bombs appeared to have been made from explosives and shrapnel packed inside a pressure cooker, law enforcement officials told NBC News. A trigger likely detonated the explosions, which officials described as a "low explosive."
"We have received voluminous tips since the incident," said Richard DesLauriers, special agent-in-charge of the FBI's Boston Field Office. "We are following up on a variety of leads, and have begun conducting interviews with the public," he said.
DesLauriers said there was, "no imminent physical threat anywhere associated with this investigation." He was joined at a Tuesday news conference by Boston Police Commissioner Edward Davis, who said nobody was currently in custody.
Law enforcement officials said nothing of value or interest was found in the Revere, Mass., home of a student who was held for questioning immediately after the explosions. The 20-year-old man was not a suspect and was recovering from burns at a Boston hospital, NBC News reported.
The FBI was working to reconstruct a timeline of events leading up to the two blasts, which killed three people—including an 8-year-old boy—and injured more than a 170 on Monday. Of those injured, 17 were in critical condition Tuesday morning. DesLauriers and his colleagues clarified that only two devices had exploded and that no unexploded devices had been recovered.
Several law enforcement officials said Monday night that the two bombs included BB's and ball bearings to be used as shrapnel, NBC News reported. Doctors from Boston hospitals said Tuesday that patients suffered excessive burns and were injured with what looked like spiked nails without heads.
The crime scene comprised the 12-block radius around the finish line, and officials expected it would take several days to process the area. Gene Marquez, with the bureau of Alcohol, Tobacco, and Firearms (ATF) said teams of explosives specialists, bomb-sniffing dogs, and residue experts were working investigating the area around Copley Square in central Boston.
Marquez called on the public to share any video or photographic evidence that could assist the investigation.
Davis called it, "the most complex crime scene," in the history of the Boston Police Department. The crime scene was geographically confined to eastern Massachusetts and the greater Boston area, DesLauriers said.
Doctors from Boston hospitals said Tuesday that patients were injured with what looked like spiked nails without heads and suffered excessive burns.
President Obama vowed Monday to bring the perpetrators to justice. "We will get to the bottom of this and we will find out who did this," Obama said.
Massachusetts Gov. Deval Patrick said there would be an interfaith prayer service held Wednesday for victims and family members.
The FBI is spearheading an investigation described by the Special Agent in Charge as a "a potential terrorist investigation."