(CNN) - Investigators are pleading for help in hunting down those responsible for a pair of bombings that left a gruesome scene at the Boston Marathon.
The blasts, which killed an 8-year-old boy and two other people, 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 up.
Exhausted marathoners had to muscle the energy to flee the bloody scene.
And more than 150 people were hospitalized, some in critical condition.
"No piece of information or detail is too small," the FBI Boston Division said, asking the public for any information or images that might provide clues.
Investigators don't know the motive for the bombings and don't have a specific suspect, nor have they found any showing the bombs being placed, a law enforcement source told CNN on Tuesday.
While authorities search for answers, many are at a loss to explain why anyone would target an annual event that celebrates thousands of runners from around the world.
A stunning attack
The blasts happened in quick succession, near the row of 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.
It also knocked 78-year-old runner 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.
At least 144 people were hospitalized, with at least 17 in critical condition and 25 in serious condition, officials said. At least eight of the patients are children.
Dr. Albert Pendleton, an orthopedic surgeon who was helping staff the race's , told CNN on Tuesday it was "basically like the bomb took out he legs of everybody."
"It was horrific," he said.
A terrorism expert briefed on the investigation said doctors had removed ball bearings from patients wounded in the bombings, suggesting the bombs were designed to propel shrapnel.
But Dr. Ron Walls of Brigham and Women's Hospital, which received 31 patients, said the debris found in some patients' wounds did not appear to be from ball bearings.
"Everything we saw was sort of ordinary ambient material that could have been propelled by the blast but was not added to the device," Walls said. "It was not the kind of things that would be added to a device to make it more injurious than it otherwise would be."
At Massachusetts General Hospital, several patients suffered from injuries to lower limbs that will require "serial operations" in the coming days, trauma surgeon Peter Fagenholz said Monday night.
He said the most serious wounds "have been combined, complex lower injuries that involve blood vessels, bone and tissue."
Numerous patients had to have limbs amputated, Fagenholz said.
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.