Updated Tuesday, April 16, 9:42 a.m.
Two explosions near the finish line of the Boston Marathon Monday killed at least three people, including an 8-year-0ld boy, and left more than 150 injured. The FBI is leading what the Special Agent in Charge described as "a potential terrorist investigation."
President Barack Obama urged caution and pledged that the perpetrators "will feel the full weight of justice."
"We still do not know who did this or why," Obama said, "and people shouldn't jump to conclusions before we have facts."
But "make no mistake, we will get to the bottom of this and we will find out who did this."
Authorities questioned a 20-year-old student for hours in a Boston hospital and searched his home in nearby Revere, Mass., NBC News reported. Officials stressed that they do not have a suspect in custody. The man was seen running from the area and had burns. Federal agents removed some materials after they searched his home, NBC's Pete Williams said.
The FBI will hold press conference at 9:30 Tuesday in Boston.
Obama said he had spoken with FBI director Robert Mueller, Homeland Security Secretary Janet Napolitano, and members of Congress.
"We do not yet have all the answers," he said, but, America "will say a prayer for Boston tonight."
Boston Police Commissioner Edward Davis said that prior to the explosion, "there was no specific intelligence that anything was going to happen." He said the area had been combed. There were abandoned packages still to be examined, but "there are no further [explosive] devices that we've found at this time."
The near-simultaneous explosions occurred 50 to 100 yards apart near the finish line before 3 p.m. EST, Davis said.
Massachusetts Representative Bill Keating said he had heard that at least two additional explosive devices had failed to detonate and had been found.
"Any event with multiple explosive devices—as this appears to be—is clearly an act of terror, and will be approached as an act of terror," said a White House official to NBC News Monday night. "However, we don't yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic."
There has not been a major terrorist attack on U.S. soil since the 9/11 attacks, in 2001, carried out by al-Qaeda, which killed about 3,000 people in New York City, Pennsylvania, and Washington, D.C.
Mark Potok, of The Southern Poverty Law Center said there was no evidence of any militia group monitored by the organization being involved. But he noted that Monday is Patriots Day, a symbolically important day to militia groups. The day marks the anniversary of the April 19, 1993, FBI raid of the Waco compound, as well as the April 19, 1995, Oklahoma City bombings.
The number of anti-government groups, or "Patriot" organizations, hit an all-time high last year, the Southern Poverty Law Center previously reported.
A separate incident occurred a short while later at the new wing of the John F. Kennedy Library in Boston, but authorities said it caused no injuries. "We are not certain these incidents are related, but we are treating them as if they are," Davis said. "It appears to have started in the mechanical room of new wing of the building," the JFK Library and Museum said in a statement. "All staff and visitors are accounted for and safe."
President Obama received a briefing on the explosions from Counter Terrorism adviser Lisa Monaco shortly after 3 p.m., White House officials said to NBC News. Obama then spoke with Boston Mayor Tom Menino and Massachusetts Governor Deval Patrick.
"The city of Boston is open...but it will not be business as usual," the governor said.
He said the blast area would remain closed and inaccessible for now. He called for vigilance and patience as law enforcement will be conducting random checks of backpacks and other parcels. FBI Special Agent in Charge Rick Deslauriers asked people to call 1-800-CALL-FBI with any information. He emphasized that "very very substantial federal resources" are being brought to bear.
Families in search of loved ones can contact 1-617-635-4520.
Boston Hospitals reported they have 126 patients from the incident as of Tuesday morning.
Marathon participant Jeff Clachko described the scene. "I literally crossed the finish line, took three steps and heard a loud explosion. Turned around and there was a mushroom, smoke mushroom--looked to be about 20 feet in the air or so. Probably about three seconds later, I heard a second one. and then, you know, realized what was going on." Clachko said "the volunteers did an incredible job ushering the runners away and they all immediately ran toward the explosion."
The last mile of the Boston Marathon, in which 27,000 runners participate every year, was dedicated this year to victims of the mass shooting at Sandy Hook Elementary School that left 26 people dead in Connecticut.
Security was increased in major metropolitan areas along the Northeast corridor. The New York Police Department increased the number of officers on duty. In Washington, D.C., Pennsylvania Avenue next to the White House was shut down.
New York City Mayor Michael Bloomberg said the NYPD resources "are being fully mobilized to protect our city."