A federal law enforcement source told Fox News that investigators are looking for two men that are persons of interest in Monday's Boston Marathon terror attack and have distributed photos for “law enforcement eyes only.”

The FBI is not sharing the photos with the public.

A Fox News reporter has seen the photos and called them “clear.”

According to the reporter, one of the men has a backpack that appears to match the bag believed to have been used in the attack. The other man also appears to have a bag of some kind.

The source asked that the images not be made public for fear of jeopardizing the investigation.

News of the images came as investigators spent Wednesday poring over photos and video footage and piecing together shredded remnants of bombs in an effort to zero in on suspects behind the attack.

Even as amateur sleuths analyzed photos of the finish-line crowd and shared their own conclusions, FBI investigators were believed to be doing the same, and there were reports that they were interested in specific images of a person or persons with heavy backpacks. Investigators believe the bombs, at least one of which was likely a pressure cooker bomb laden with explosives and shrapnel, may have been concealed in such a bag. Monday's attack killed three and injured 176, many critically.

A mangled pressure cooker lid found atop a nearby building is believed to have been part of one bomb, and it and other pieces were being analyzed at an FBI lab. A battery and several pieces of shrapnel were also recovered and undergoing analysis. Fox News learned that the circuit board suspected of being used to detonate at least one of the bombs has been recovered, and that FBI investigators were also analyzing cellphone tower records to identify positive hits for signs of calls that may have been placed to trigger both explosions remotely.

The FBI and Boston Police said no arrests have been made in the Boston Marathon bombing. Earlier, FoxNews.com and several other media outlets, including The Associated Press, reported that a suspect had been arrested in the deadly attack.

Investigators believe one, or possibly two, pressure cookers were packed with explosives and shrapnel and hidden in backpacks to be left amid the crowd.

According to a FBI and Department of Homeland Security bulletin, the deadly shrapnel that caused the deaths -- including of an 8-year-old boy, and critical injuries to 17 -- included nails, BBs and ball bearings. The other device "was also housed in a metal container, but investigators could not say if that was also a pressure cooker.

Pressure cooker bombs have been used in high-profile bombings in India, Pakistan and Afghanistan, and have been touted in the Al Qaeda in Yemen's online propaganda magazine Inspire and in the "The Anarchist Cookbook."

The horror unfolded just before 3 p.m., shattering a festive atmosphere several hours after the legendary race began on the city's 238th annual Patriots' Day. In the aftermath, officials found bomb remnants, shrapnel and shredded backpacks believed to have concealed the deadly payloads.

Richard DesLauriers, FBI agent in charge in Boston, confirmed at a press conference investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker. He said the items were sent to the FBI laboratory at Quantico, Va., for analysis.

The FBI also plans to reconstruct the devices at their headquarters in Quantico, according to MyFoxBoston.com.

Investigators are also examining if the bombs could have been assembled near the scene of the explosions, The Wall Street Journal reports, quoting a law enforcement official. The official says this possibility is being considered because transporting improvised devices over a significant distance could trigger a premature detonation.

The officials also say federal agents are working with local police to canvas local hotels and short-term rentals for clues as to where the bombs could have been constructed.

Dr. Peter Burke, chief of trauma services at Boston Medical Center, said Wednesday 19 patients (six males and 13 females) are being treated at the facility. Seven people had amputations and two remain in critical condition, including a 5-year-old boy. He said there will be eight 're-operations' at the hospital.One or two patients may be released later today. He credited Boston EMS and first responders for limiting the loss of life.

He said beside amputations, doctors have been treating lung injuries, likely caused from the force of the explosion or impact on the ground.

"The major (injuries) ones that were life-treating were tissue injuries and vascular injuries," he said.

Tufts Medical Center said it treated 14 patients, seven of whom have been discharged. None of the patients are listed in critical condition. The hospital said it did not have any amputations nor does it expect any in the future.

"This is something I've never seen in my 25 years here," Alasdair Conn, chief of emergency services at Massachusetts General Hospital said Monday night; "this amount of carnage in the civilian population. This is what we expect from war."

An intelligence bulletin issued to law enforcement and released late Tuesday includes a picture of a mangled pressure cooker and a torn black bag, like the one seen below that was first obtained by MyFoxAtlanta.com, that the FBI says were part of a bomb.

It remained unclear if the bombs were the work of a homegrown or foreign threat, but in Washington, both President Obama and Defense Secretary Chuck Hagel called the attack terrorism.

“Any time bombs are used to target innocent civilians, it is an act of terror,” Obama said Tuesday morning.

Boston Police Commissioner Ed Davis appealed to the public to come forward with any information or photographic evidence that might help authorities zero in on the killers.

The Pakistani Taliban, which has threatened attacks in the U.S. because of its support for the Pakistani government, denied any role in the marathon bombings Tuesday.

The group's spokesman, Ahsanullah Ahsan, denied involvement in a telephone call with The Associated Press. He spoke from an undisclosed location.

Details also emerged Tuesday on the victims of the attack, as scores of victims remained in hospitals, many with grievous injuries.

Eight-year-old Martin Richard, who was with his family cheering on runners at the finish line, is among those dead. The boy's 6-year-old sister lost a leg in the attack and his mother suffered a brain injury.

Krystle Campbell, of Medford, Mass., 29, was also killed in the blast. Her father said she had gone with her best friend to take a picture of the friend's boyfriend crossing the finish line.

William Campbell said his daughter, who worked at a restaurant in nearby Arlington, was "very caring, very loving person, and was daddy's little girl." He said the loss has devastated the family.

The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi. She was a graduate student at Boston University.

The first two explosions occurred at 2:50 p.m. – nearly five hours after the marathon began – about 50 to 100 yards apart, according to Davis. A third explosion occurred near the John F. Kennedy Presidential Library and Museum in the Columbia Point section of Dorchester, several miles southeast of the marathon's finish line, at around 4:15 p.m. Police could not say if it was related to the earlier explosions.

Nearly 25,000 people, including runners from around the world, competed in Boston's celebrated 26.2-mile race, attracting huge throngs of onlookers, especially near the finish line.

Obama plans to visit Boston on Thursday to attend an interfaith service in honor of the victims. He has traveled four times to cities reeling from mass violence, most recently in December after the schoolhouse shooting in Newtown, Conn.

Anyone with information on the bombings is being urged to call Boston authorities at 1-800-494-TIPS.

Fox News' Rick Leventhal, Jana Winter, Catherine Herridge and Mike Levine contributed to this report.