(CNN) -- Investigators scrambling to solve the fatal bombing at the Boston Marathon have made "significant progress" in the case but no arrests, a federal law enforcement source told CNN's John King Wednesday.
The FBI also officially confirmed in a statement that no arrests had been made.
The denials came after sources previously told CNN that a suspect was in custody.
One federal law enforcement source told CNN that "anyone who says 'arrest' is ahead of themselves."
A Boston law enforcement told CNN, "We got him," but won't clarify whether that means a suspect has been identified or arrested.
Some federal sources said it was even too early to say investigators had identified the suspect, but several sources in Boston told CNN that they have a clear identification.
Men in hazardous materials suits investigate the scene at the first bombing site on Boylston Street in Boston. (Elise Amendola / Los Angeles Times, April 16, 2013)
The back-and-forth developments came after a chaotic day in which investigators revealed more details about the makeup of the bombs and apparently unrelated scares over letters containing ricin gripped the nation's capital.
The bombs exploded 12 seconds apart near the finish line of the Boston Marathon, killing three people and wounding about 180 others.
One of the bombs 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.
While the clues moved the investigation forward, they did not make it immediately apparent 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."
Things we know
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."
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Third victim identified
Boston University identified graduate student Lingzu Lu as the third person who died in Monday's bombings.
Previously identified were Krystle Campbell, 29, of Arlington, Massachusetts, and Martin Richard, 8, of Dorchester, Massachusetts.
"She was the best," Campbell's distraught mother, Patty, told reporters Tuesday. "You couldn't ask for a better daughter."
Martin "was a bright, energetic young boy who had big dreams and high hopes for his future," his school said in a statement. "We are heartbroken by this loss."
The hunt for the attacker
The attack 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 sifted through thousands of pieces of evidence and a mass of digital photos and video clips leading up to Wednesday's arrest. They had pleaded 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 said. "The community will play a crucial role in this investigation."
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.
'Human spirit' still alive
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.
Victims continue recovery
As investigators closed in on a suspect, those wounded in the incident continued to recover.
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.
The incident deeply affected thousands, including Candace Rispoli, who was cheering on a friend when the festive atmosphere turned into a "terrifying hell." She suffered minor injuries.
"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.