"Any time bombs are used to target civilians, it is an act of terrorism," Obama said. While it remained unclear who carried out the attack and why, the Federal Bureau of Investigation (FBI) was probing it as a terror act, he said.
Obama said that "this is a good time for all of us to remember that we all have a part to play in alerting authorities if you see something suspicious", telling Americans to "speak up".
Heordered the flag above the White House lowered to half-staff as he huddled inside with top advisers, including Vice President Joe Biden, FBI Director Robert Mueller, Attorney General Eric Holder and Homeland Security Secretary Janet Napolitano.
Cabinet members, along with national security and counterterrorism staff, will again brief the president Tuesday evening at the White House about the bombing at Boston Marathon, the world's oldest annual marathon, dating back to 1897.
Authorities warned Boston residents to keep their guard up Tuesday amid a massive investigation to uncover a clear reason for the twin blasts that went off near the Maraton finish line Monday afternoon killing three, including an 8-year-old boy.
Of the 176 people who were treated at hospitals, at least 17 were in critical condition and 41 in serious condition, CNN reported citing hospital officials .
At least nine of the wounded were children. Some of the wounded kids have already left the hospital, Boston Children's Hospital spokeswoman Meghan Weber was quoted as saying.
Dr. Albert Pendleton, an orthopaedic surgeon who was helping staff at the race's medical tent, told CNN on Tuesday it was "basically like the bomb took out the legs of everybody".
"It was horrific," he said.
Investigators spent Monday going over the 12-block crime scene and fanning out to interview witnesses.
Law enforcement hasn't come to any conclusion about whether the bombing was the work of domestic or international terrorists, CNN said citing two senior Pentagon officials.
Investigators don't know the motive for the bombings and don't have a specific suspect, nor have they found any surveillance video showing the bombs being placed, the channel said citing a law enforcement source.
The blasts happened in quick succession, near the row of international flags that led up to the finish line. The impact was so powerful, it whipped the limp flags straight out, as if they were caught in a hurricane.
Some runners said they thought the first blast was from a celebratory cannon. But "When the second one happened, it was very 9-11ish," runner Tom Buesse told CNN's "Starting Point" Tuesday.
One blast knocked 78-year-old marathoner Bill Iffrig to the ground.
"I was just approaching the last straightaway to the finish line, and I had a good day and was feeling really good, and I got down to within about 15 feet of the finishing apron and just tremendous explosion, sounded like a bomb went off right next to me," Iffrig was quoted as saying.
Iffrig was not seriously injured. But trails of blood, severed arms and legs and other body parts littered the scene nearby. Several patients lost limbs, doctors said Tuesday in briefings.
Confusion reigned Tuesday over whether the bombs contained ball bearings or some other form of shrapnel -- a key indicator that could help investigators fingerprint the explosives and find those who made them, CNN said.
Doctors at Massachusetts General Hospital found pellets and sharp nail-like objects in their patients who were wounded in the bombings, said Dr. George Velmahos, head of trauma care for the hospital.
A federal law enforcement official cited by CNN said both bombs were small, and initial tests showed no C-4 or other high-grade explosive, suggesting that the packages used in the attack were crude.
The explosions went off near the finish line about 4 hours and 9 minutes into the race, within a 10-minute window of the average finish time for the marathon.
Officials have no suspect in custody, but many people are being questioned, Boston Police Commissioner Ed Davis said.