(CNN) - A day after the fatal terror attack on the Boston Marathon, authorities warned the city's residents to keep their guard up Tuesday amid a massive investigation to uncover a clear reason for the bombings.
Three people died -- including an 8-year-old boy -- and more than 170 were injured when the twin blasts went off near the finish line Monday afternoon.
"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," President Barack Obama said Tuesday after a briefing with his national security team. "Anytime bombs are used to target innocent civilians, it is an act of terror."
Boston leaders vowed to emerge from the attack unbowed.
"Moments like this and our response to them define who we are," Suffolk County District Attorney Dan Conley said Tuesday.
"Boston will overcome," Mayor Thomas Menino promised.
Investigators spent Monday going over the 12-block crime scene and fanning out to interview witnesses, with FBI Boston Field Office Special Agent in Charge Richard DesLauriers vowing to go to the "ends of the earth" to find out who was behind the bombing.
Law enforcement hasn't come to any conclusion about whether the bombing was the work of domestic or international terrorists, two senior Pentagon officials told CNN on Tuesday.
Despite earlier reports that more bombs had been found, Massachusetts Gov. Deval Patrick said there were no explosives other than the two that detonated. DesLauriers said authorities were aware of no new public safety threats, but police officials asked Boston residents for patience with swarming investigators and increased security precautions around the city.
They also pleaded for the public to submit cell phone images and video that could help unravel the mystery of who created such carnage at one of the nation's most storied sporting traditions.
The blasts marked a grotesque end to what should have been a celebration of triumph.
One man's legs were instantly blown off, yet he kept trying to stand. Exhausted marathoners had to muscle the energy to flee the bloody scene.
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, a law enforcement source told CNN on Tuesday.
A day after the bombings, as Pope Francis told Bostonians to "combat evil with good," and runners in Atlanta staged a silent run to commemorate the victims, Americans alternately mourned and nervously wondered who was behind the violence.
A stunning attack
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. Any such illusions were shattered when the second blast erupted, startling the exhausted runners out of their post-race daze.
"When the second one happened, it was very 9/11-ish," runner Tom Buesse told CNN's "Starting Point" on 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 said.
"The shock waves just hit my whole body and my legs just started jittering around. I knew I was going down," he said.
Iffrig was not seriously injured. But trails of blood, severed arms and legs and other body parts littered the scene nearby.
Of the 176 people who were treated at hospitals, at least 17 were in critical condition and 41 in serious condition, according to 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 said.
Dr. Albert Pendleton, an orthopedic surgeon who was helping staff 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.
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.
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.
But earlier, a federal law enforcement source discounted reports the bombs had been packed with ball bearings. The source told CNN National Security Analyst Fran Townsend that investigators believe the bombs may have been placed in a trash can, fragments of which turned into shrapnel when they exploded.
Another federal law enforcement source with knowledge of the investigation said one bomb appeared to have been placed inside a metal pressure cooker hidden in a backpack.
The hunt for clues
Investigators don't know who was behind the attack, or whether it was spawned domestically or from afar. But federal authorities are classifying it as an act of terrorism.
Federal and local investigators -- including bomb technicians -- searched an apartment in nearby Revere, the city's fire department posted on Facebook early Tuesday.
The search is linked to a young Saudi Arabian man in the United States on a student visa, the law enforcement source who spoke to CNN on Tuesday said.
Nothing was found at the apartment linking the man to the bombings, the source said, cautioning not to read too much into the search as investigators will be talking to numerous people.
The search took place by consent, according to another federal law enforcement source, meaning no search warrant was needed.
A federal law enforcement official 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.
Obama, who said Monday the bomber or bombers would "feel the full weight of justice," received overnight briefings on the investigation, a White House official said.
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.
But the fact that the blasts took place near the end of the race "seems to indicate this was not geared toward maximum damage," said a former federal law enforcement official who now works in the intelligence community.
"It may speak volumes about the (level of) planning that went into this," the source said. "It raises questions ... why didn't the bombs go off when the crowd was packed in like sardines when the winners were crossing the finish line? It could mean the people behind it couldn't get access to the area when they originally intended."
Officials have no suspect in custody, but many people are being questioned, Boston Police Commissioner Ed Davis said.
Investigators warned police to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with the attack, according to a law enforcement advisory obtained by CNN. The man was seen with a black backpack and sweatshirt and was trying to get into a restricted area about five minutes before the first explosion, the lookout notice states.
'Just like going back' to Iraq
Nurses Stephen Segatore and Jim Asaiante were stationed near the finish line, expecting to treat the usual ailments from runners -- cramps and dehydration.
Suddenly, they found themselves in a battlefield, with blood and debris everywhere.
"For me, it was just like going back to being in Iraq in 2006-2007," said Asaiante, an Army captain who served an 18-month tour.
"I heard the first IED, and I know there's never one. The bad guys always set up two or three" improvised explosive devices, he said.
The plumes of smoke and images of bloodied victims running down streets also triggered haunting memories of the terrorist attacks of 9/11.
Tami Hughes had just crossed the finish line and was looking for her husband when she heard a deafening explosion.
"I didn't know if it was a small aircraft going into the building," said Hughes, who was in the bustling financial district during the 9/11 attacks.
"I turned around and immediately saw the whitish-brownish smoke billowing up four or five stories and I couldn't believe that, you know, could it be a bomb? And I stared at it and about five or seven seconds later, when the second one went off, I knew immediately that it was something coordinated or organized."
Never the same again
The Boston Marathon is the world's oldest annual marathon, dating back to 1897.
It's a tradition that not only symbolizes the arrival of spring in Boston, and it also marks Patriots Day, which commemorates the day of the opening battle of the Revolutionary War.
Each year, more than 20,000 pound the pavement through the winding streets of Boston as thousands of spectators cheer them on.
After Monday's tragedy, some wondered whether the spectacle would ever happen again.
"The Boston Marathon has endured two world wars and many other things," said Fred Treseler, who has helped train more than 3,000 athletes for the race.
"I am quite sure there will be a Boston Marathon next year. But for certain, the Boston Marathon has been changed forever."