UPDATE: Boston University graduate student Lingzi Lu has been identified by the school as the third person who died in Monday's bombings at the Boston Marathon.
Within a day of the Oklahoma City bombing, officials had named their suspect: Timothy McVeigh. Within two days of the 9/11 attacks, investigators had zeroed in on al Qaeda as the perpetrator.
But as loved ones mourn the deaths of three people and dozens of others remain hospitalized from the dual bombings Monday near the finish line of the Boston Marathon, two questions continue to hound authorities: Who triggered the attack, and why?
On Wednesday morning, a federal law enforcement source with firsthand knowledge of the investigation told CNN that a lid to a pressure cooker thought to have been used in the bombings had been found on a roof of a building near the scene.
While such clues may move the investigation forward, they did not reveal whether the attack was an act of domestic or foreign terrorism.
"If your experience and your expertise is Middle East terrorism, it has the hallmarks of al Qaeda or a Middle East group," former FBI Assistant Director Tom Fuentes said. "If your experience is domestic groups and bombings that have occurred here, it has the hallmarks of a domestic terrorist like Eric Rudolph in the 1996 Atlanta Summer Olympics bombings."
Fuentes said he has investigated both types of terrorism -- from Iraq to the United States -- and finds the Boston attack has elements of both. "It has the hallmarks of both domestic and international (attacks), and you can see either side of that."
The bombs, which exploded 12 seconds apart, killed three people and wounded 183.
One was housed in a pressure cooker hidden inside a backpack, the FBI said in a joint intelligence bulletin. The device also had fragments that may have included nails, BBs and ball bearings, the agency said.
The second bomb was also housed in a metal container, but it was not clear whether it too was in a pressure cooker, the FBI said.
The U.S. government has warned federal agencies in the past that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel and detonating them with blasting caps.
Photos obtained by CNN show the remains of a pressure cooker found at the scene, along with a shredded black backpack and what appear to be metal pellets or ball bearings.
Scraps of at least one pressure cooker, nails and nylon bags found at the scene were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices, the agent leading the investigation said Tuesday.
The pieces suggest each of the devices was 6 liters (about 1.6 gallons) in volume, a Boston law enforcement source said. The recovered parts include part of a circuit board, which might have been used to detonate a device.
A law enforcement official said Monday's bombs were probably detonated by timers. But the FBI said details of the detonating system were unknown.
The hunt for the attacker
The attack has left Boston police with "the most complex crime scene that we've dealt with in the history of our department," Commissioner Ed Davis said Tuesday.
Authorities are sifting through more than 2,000 tips and a mass of digital photos and video clips, but are asking for the public's help in providing additional leads and images.
"Someone knows who did this," said Rick DesLauriers, the special agent in charge of the FBI's Boston office. "Cooperation from the community will play a crucial role in this investigation."
DesLauriers asked the public to report anyone who may have talked about targeting the marathon or shown interest in explosives. He urged anyone who may have heard explosions in remote areas -- possibly indicating a bomb test -- or seen someone carrying "an unusually heavy, dark-colored bag" around the time of the attack to come forward.
Who may have wanted to cause mass destruction -- and whether the individual or individuals acted alone or as part of a group -- remained a mystery.
"We really don't know if it's a foreign or domestic threat," said Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee. "We don't know whether this was a homegrown terrorist or part of a wider conspiracy."
A "lone wolf" attacker could be particularly difficult to identify.
"This is what you worry about the most," a source with knowledge of the investigation said. "No trail, no intelligence."
Clues from inside the hospital
Medical personnel treating the wounded found evidence suggesting the bombmaker or bombmakers sought to maximize the suffering.
Dr. George Velmahos, head of trauma care at Massachusetts General Hospital, said his team found metal pellets and nails inside patients' bodies.
"They are numerous. There are people who have 10, 20, 30, 40 of them in their body, or more," Velmahos said.
While most of the patients treated at Brigham and Women's Hospital were wounded by "ordinary debris," three were struck by "perfectly round objects" that were uniform, consistent and metallic, the hospital's chairman of emergency medicine said.
Dr. Ron Walls also said one patient had more than 12 carpenter-type nails.
"There is no question some of these objects were implanted in the device for the purpose of being exploded forward," he said.
The process of healing is already well under way. Boston Medical Center has two patients in critical condition, down from 11 just after the bombings, Dr. Peter Burke, chief of trauma care, told reporters Wednesday. Ten patients are in serious condition and seven are in fair condition, he said.
A traumatic road ahead
Candace Rispoli was cheering on a friend when the festive atmosphere turned into a "terrifying hell."
"One of my best friends ... was actually trampled and thrown to the blood-splattered ground when it first happened," Rispoli told CNN's iReport. "She was able to get up and keep running, but when I turned around to seek all my friends, I could not see her and panicked."
Rispoli, who suffered minor injuries, said the attack has changed her life.
"I personally will never participate in an event of this nature in a city in fear that something like this could happen again," she said. "I keep replaying the moments of terror over and over in my head and am just still in utter shock. Always seeing terrible things of this nature happen all over the world on TV, my heart would always go out to those directly affected. But I never imagined in a million years I would be a spectator at the Boston Marathon running for my life."
CNN's Fran Townsend, Matt Smith, Dave Alsup, Henry Hanks, Susan Candiotti, Rande Iaboni, Gloria Borger and John King contributed to this report.
PHOTO: Courtesy of Aaron Tang