(CNN) -- Beyond the shattered glass, the blood, the wails of pain, there are questions: Who did this? Why? And how?
Monday's terror attack on the Boston Marathon killed an 8-year-old boy watching with his family, a 29-year-old woman loved by her family and friends and a Boston University graduate student. More than 180 others were wounded, many losing limbs as a result of horrific twin blasts near the race's finish line, in the heart of the city.
A full day later, authorities are still grappling for answers. No suspect had been named. No one has claimed responsibility.
All Richard DesLauriers, the special agent in charge of the FBI's Boston office, could say -- beside that it's believed BBs and nails were part of the explosive devices, which likely had been put in black nylon bags or backpacks -- is that the more than 1,000 law enforcement authorities will go to the "ends of the Earth" to find the culprits.
"The range of suspects and motives remains wide open," DesLauriers said late Tuesday afternoon.
For some, the attacks brought immediate comparisons to the September 11, 2001, attacks, partly because of reports that investigators talked with Saudi Arabian citizens who were at the site -- even though a U.S. official said one Saudi male questioned was simply in the "wrong place at the wrong time."
For others, the bombs that exploded near the finish line of the marathon Monday afternoon felt more homegrown, more akin to the April 1995 Oklahoma City bombing that left 168 people dead or the 1996 Atlanta Olympics bombing that killed two.
Authorities took pains to caution Americans against jumping to conclusions.
Combing through the debris, fanning out to interview witnesses, scanning radical websites, they were ruling nothing out -- or in -- on Tuesday.
President Barack Obama will attend an interfaith service Thursday morning at the Church of the Holy Cross, a Catholic Church in Boston's South End. On Tuesday, he met with members of his national security team and seemed to signal that investigators are starting from scratch.
They don't know, he said, whether the bombing was a work of an organized group or a disgruntled loner. Nor do they know whether the violence came from within or was imported from overseas.
"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 terror," he said.
The blasts Monday happened in quick succession in Boston's historic Copley Square, near the row of international flags leading to the finish line. The pressure wave 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 said on CNN's "Starting Point" on Tuesday.
As details emerged about the type of explosives used in the attacks, so did the intimate details of the victims' lives.
An image of 8-year-old Martin Richard shows the boy holding a bright blue sign with the word "Peace." On Monday, the boy and his family were watching the Boston Marathon near the finish line when the two bombs exploded, killing him.
Krystle Campbell, who graduated from Medford High School in Massachusetts in 2001, also died in the attack.
Her mother, Patty Campbell, tearfully described her daughter as "the best," recalling, from the steps of the family's Medford home, that Krystle was "always smiling" and had a "heart of gold."
"She was a fun, outgoing person," the girl's grandmother Lillian Campbell told CNN, adding that the family learned Krystle was dead on Tuesday. "She was always there to help somebody. And she was just beautiful."
The third victim was a Boston University grad student, who has not been named, the school said. In a statement, Boston University said the student had been watching the race near the finish line with two others when the explosives detonated.
One of the three students was unharmed, but another -- a woman -- is in stable condition at Boston Medical Center after undergoing surgeries each of the past two days.
As is often the case in such incidents, rumors of other bombs quickly began to circulate -- around Massachusetts and the country.
In the Bay State, Gov. Deval Patrick and federal investigators insisted the only known explosives were the two bombs that went off and did not detail any new threats.
But there were scares elsewhere. Jittery passengers on a flight from Boston to Chicago on Tuesday, for instance, expressed concern over two Arabic-speaking passengers, who were removed from the plane.
And security scares disrupted activities at Boston's Logan International Airport, LaGuardia Airport in New York and a train station in Cleveland.
All were unfounded, authorities said.
In Boston, police urged residents to be patient with the extra security precautions in transit stations and elsewhere. Investigators checked vents and pipes of buildings near the site of the explosions, according to a law enforcement source on the scene.
Meanwhile, local leaders promised to emerge unbowed.
"Boston will overcome," Mayor Thomas Menino said.
Although investigators still don't know who was behind the bombing, they moved closer Tuesday to understanding the devices used.
DesLauriers said the bombs were possibly placed in a pressure cooker hidden inside a backpack or another black nylon bag.
Another law enforcement official told CNN it was "likely but not certain" that the bombs were on a timer and not set off remotely by a cell phone.
Another 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.
Two photos -- taken by the same person and obtained by CNN affiliate WHDH -- show a light-colored bag on the sidewalk next to a mailbox and then the scene immediately afterward. The second photo shows a blast right around where that bag was -- it is no longer visible -- with the mailbox still standing.
These photos have been provided to investigators, WHDH reports. Like many things about the investigation, it's not clear if one of the bombs originated in such a bag. However,the FBI has said the bombs were put in dark-colored nylon bags.
Investigators have not commented on the photos.
Whatever they were in, there is no doubt the explosives were powerful -- and deadly.
Dr. Ron Walls, the emergency medicine chairman at Boston Brigham and Women's Hospital, said surgeons there had retrieved something like nails, as well as "consistent, metallic, perfectly round objects" slightly larger than a BB, from victims.
"There is no question that ... some of these objects were planted in the device for the purpose of being exploded forward when the bomb went off," he said.
Search for answers
Authorities pleaded for patience with swarming investigators, who expect to occupy Copley Square for at least two days, snarling traffic and interrupting the lives and work of countless Bostonians.
They asked the public to submit photos and videos that could help identify a suspect or explain how someone was able to slip bombs undetected into what Police Commissioner Ed Davis described as one of the most photographed spots in America.
"Our mission is clear: to bring to justice those responsible for the marathon bombing," said the FBI's DesLauriers. "The American public wants answers; the citizens of the city of Boston and the commonwealth of Massachusetts want and deserve answers."
Late Monday, authorities began searching the apartment of a young Saudi man who was on a student visa and injured at the race in nearby Revere. He was found to have no connection to the attack. "He was just at the wrong place at the wrong time," a U.S. official told CNN.
A Saudi woman, a medical student who was also injured in the blast, has also been interviewed by investigators, according to a law enforcement source.
Davis said many people were being questioned.
Like "being in Iraq"
Nurse Jim Asaiante was stationed near the finish line, expecting to treat the usual ailments from runners: cramps and dehydration.
Suddenly, he found himself 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," he said.
Dr. Albert Pendleton, an orthopedic surgeon who was helping staff the race's medical tent, said Tuesday that 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.
Eleven Boston-area hospitals treated 183 injured patients -- 23 of whom were at one point in critical condition and 40 of whom were in serious -- hospital officials told CNN. Nine children were among them. At least 89 had gone home as of late Tuesday afternoon, according to a CNN tally.
One blast knocked 78-year-old marathoner Bill Iffrig to the ground.
"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.
Never the same again
After Monday's tragedy, authorities in New York and Los Angeles stepped up security while those in London began reviewing measures for that city's upcoming marathon.
Back in Massachusetts, some wondered what would happen to the Boston Marathon. The Boston Athletic Association, which runs the event, said Tuesday that it is "committed to continuing the tradition" next year. And Gov. Patrick vowed Tuesday, "Next year's marathon will be even bigger and better."
But there was no doubt things had changed.
The race -- which draws more than 20,000 participants -- is the world's oldest annual marathon, dating to 1897.
It's a tradition that not only symbolizes the arrival of spring in Boston, it marks Patriots Day, which commemorates the day of the opening battle of the Revolutionary War.
"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."