Story highlights Authorities ask federal and state agents to help identify two men seen in photos

The men are of "high interest" in the probe into the Boston Marathon blasts

One of the men is seen in photos carrying a black backpack, an official says

3 people were killed and about 180 injured when 2 bombs went off near the finish line

Investigators have pinpointed two men as "possible suspects" who were seen in images near the finish line of this week's Boston Marathon -- moments before twin bombs there exploded, killing three and injuring about 180 others -- a law enforcement official said.

A circular sent out Wednesday to federal and state agencies features the photos "in an attempt to identify the individuals," who were described as being of "high interest" to investigators.

One of the men is seen carrying a black backpack. An FBI official earlier said that authorities believe the bombs were placed inside a black nylon backpack or bag.

The source said that authorities had not yet identified the two men by name and that the photographs were not being released to the public for fear of impeding the investigation.

Earlier Wednesday, two official sources with knowledge of the investigation identified a man -- who also hasn't been named -- as a possible suspect in the attack.

Seen on a video, this man wore a white baseball cap. One of the sources added that the cap was on backwards and the man was also wearing a light-colored hooded sweatshirt and a black jacket.

It was not immediately known if this man is one of those alluded to in the photographs distributed to law enforcement officials.

This movement in the investigation came two days after the horrific blast shook the city, during what is traditionally one of its biggest and most beloved events each year.

By Wednesday, authorities had made "significant progress" in the case but no arrests, a federal law enforcement source told CNN's John King.

Sources previously told CNN that a suspect was in custody, but both Boston police and the FBI denied that any arrests had been made.

A Boston law enforcement source told CNN, "We got him," but wouldn't clarify whether that meant 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.

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Speaking on CNN's The Situation Room, Massachusetts Gov. Deval Patrick said investigators were closer to cracking the case "every hour." But he urged patience with the probe.

"What I would say and I would ask of everyone is we give law enforcement the space to do their work," Patrick said. "When they are ready with a complete picture, they will tell us what that picture is."

He added, "I wish they had nailed the perpetrator within minutes of this catastrophe, but I understand from experience it's going to take some time."

Earlier Wednesday, investigators revealed more details about the makeup of the bombs, which exploded 12 seconds apart. One had been 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 lid of a pressure cooker thought to be used in the device was found on a rooftop at the scene, a federal law enforcement official with firsthand knowledge of the investigation told CNN.

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.

The bombs

Photos: Evidence photos from Boston bombings Photos: Evidence photos from Boston bombings Evidence photos from Boston bombings – A police forensics team examines a boat April 22, 2013, in Watertown, Massachusetts, where Boston Marathon bombing suspect Dzhokhar Tsarnaev was discovered several days earlier and taken into custody. Hide Caption 1 of 15 Photos: Evidence photos from Boston bombings Massachusetts State Police released thermal images of Tsarnaev hiding in the boat on April 19, 2013. They were taken by an infrared device on a helicopter. The first image was taken at 7:19 p.m., less than 20 minutes after a homeowner told police there was a bloodied person in his boat. Hide Caption 2 of 15 Photos: Evidence photos from Boston bombings This image from 7:22 p.m. shows a white heat signature large enough to be a person. Hide Caption 3 of 15 Photos: Evidence photos from Boston bombings A robotic arm tears away the cover on the boat at 7:36 p.m. Hide Caption 4 of 15 Photos: Evidence photos from Boston bombings The heat signature clearly shows the suspect's feet and the rest of his body behind the boat console at 8:01 p.m., minutes before he surrendered. Hide Caption 5 of 15 Photos: Evidence photos from Boston bombings Boston Marathon bomb scene pictures, taken by investigators, show the remains of an explosive device. Hide Caption 6 of 15 Photos: Evidence photos from Boston bombings 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. Hide Caption 7 of 15 Photos: Evidence photos from Boston bombings One bomb was housed in a pressure cooker hidden inside a backpack, the FBI said in a joint intelligence bulletin. Hide Caption 8 of 15 Photos: Evidence photos from Boston bombings The device also had fragments that may have included nails, BBs and ball bearings, the FBI said. Hide Caption 9 of 15 Photos: Evidence photos from Boston bombings The recovered parts include part of a circuit board, which might have been used to detonate a device. Hide Caption 10 of 15 Photos: Evidence photos from Boston bombings A law enforcement official said the bombs were probably detonated by timers. But the FBI said details of the detonating system were unknown. Hide Caption 11 of 15 Photos: Evidence photos from Boston bombings Scraps of at least one pressure cooker, nails and nylon bags were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices, the agent leading the investigation said. Hide Caption 12 of 15 Photos: Evidence photos from Boston bombings 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. Hide Caption 13 of 15 Photos: Evidence photos from Boston bombings The pieces suggest each of the devices was 6 liters (about 1.6 gallons) in volume, a Boston law enforcement source said. Hide Caption 14 of 15 Photos: Evidence photos from Boston bombings "It has the hallmarks of both domestic and international (attacks), and you can see either side of that," former FBI Assistant Director Tom Fuentes told CNN. Hide Caption 15 of 15

Photos: Deadly attack at Boston Marathon Photos: Deadly attack at Boston Marathon An injured man is loaded into an ambulance after two bombs went off near the finish line of the Boston Marathon on April 15, 2013. Three people were killed and at least 264 were injured. Hide Caption 1 of 55 Photos: Deadly attack at Boston Marathon A man comforts a victim on the sidewalk at the scene of the first explosion. Hide Caption 2 of 55 Photos: Deadly attack at Boston Marathon The second explosion goes off near the finish line. Hide Caption 3 of 55 Photos: Deadly attack at Boston Marathon The first explosion knocked down 78-year-old runner Bill Iffrig at the finish line. He got up a few minutes later and finished the race. Hide Caption 4 of 55 Photos: Deadly attack at Boston Marathon A runner embraces another woman on the marathon route near Kenmore Square. Hide Caption 5 of 55 Photos: Deadly attack at Boston Marathon Former New England Patriots offensive lineman Joe Andruzzi carries a woman from the scene. Hide Caption 6 of 55 Photos: Deadly attack at Boston Marathon A victim of the first explosion is helped on the sidewalk of Boylston Street. Hide Caption 7 of 55 Photos: Deadly attack at Boston Marathon An injured person is taken away from the scene in a wheelchair. Hide Caption 8 of 55 Photos: Deadly attack at Boston Marathon A woman kneels and prays at near the finish line. Hide Caption 9 of 55 Photos: Deadly attack at Boston Marathon People run down Exeter Street after the blasts. Hide Caption 10 of 55 Photos: Deadly attack at Boston Marathon The explosions occurred around 2:45 p.m., about an hour after the first of the race's nearly 27,000 runners had crossed the finish line. Hide Caption 11 of 55 Photos: Deadly attack at Boston Marathon Victims lie on the ground at the scene of the first explosion. Hide Caption 12 of 55 Photos: Deadly attack at Boston Marathon A couple runs from the scene pushing a stroller. Hide Caption 13 of 55 Photos: Deadly attack at Boston Marathon A runner reacts near Kenmore Square after the explosions. Hide Caption 14 of 55 Photos: Deadly attack at Boston Marathon A man's blood-stained feet hang outside an ambulance. Hide Caption 15 of 55 Photos: Deadly attack at Boston Marathon A bystander who was injured in the first explosion is wheeled across the finish line while receiving medical attention from rescue workers. Hide Caption 16 of 55 Photos: Deadly attack at Boston Marathon Carlos Arredondo was at the race handing out American flags to spectators. After the blasts, he helped emergency responders and is credited with helping a man survive serious leg wounds. Hide Caption 17 of 55 Photos: Deadly attack at Boston Marathon The second explosion goes off near the finish line. Hide Caption 18 of 55 Photos: Deadly attack at Boston Marathon Police look at blown-out windows. The bombs shook buildings, witnesses said, sending people to seek shelter under tables. Hide Caption 19 of 55 Photos: Deadly attack at Boston Marathon Unclaimed runners' bags fill an area near the marathon finish. Hide Caption 20 of 55 Photos: Deadly attack at Boston Marathon Women desperate to hear from loved ones are unable to get close to the site of the attack. Hide Caption 21 of 55 Photos: Deadly attack at Boston Marathon Police and runners stand near Kenmore Square after the attack. Hide Caption 22 of 55 Photos: Deadly attack at Boston Marathon Runners gather near Kenmore Square after the explosions. Hide Caption 23 of 55 Photos: Deadly attack at Boston Marathon An EMT worker is transferred to an ambulance outside a medical tent in Copley Square. Hide Caption 24 of 55 Photos: Deadly attack at Boston Marathon A SWAT team arrives on the scene. Hide Caption 25 of 55 Photos: Deadly attack at Boston Marathon A runner sits near Kenmore Square after the attack. Hide Caption 26 of 55 Photos: Deadly attack at Boston Marathon Runners who had not yet finished the race are stopped after the explosions. Hide Caption 27 of 55 Photos: Deadly attack at Boston Marathon Victims are helped at the scene of the first explosion. Hide Caption 28 of 55 Photos: Deadly attack at Boston Marathon A man in tears is helped at the scene on Boylston Street. Hide Caption 29 of 55 Photos: Deadly attack at Boston Marathon Passersby put pressure on a victim's leg to try to stop the bleeding. Hide Caption 30 of 55 Photos: Deadly attack at Boston Marathon Massachusetts State Police guard an area near Kenmore Square. Hide Caption 31 of 55 Photos: Deadly attack at Boston Marathon Bystanders embrace near the finish line. Hide Caption 32 of 55 Photos: Deadly attack at Boston Marathon Police officers gather on Newbury Street. Hide Caption 33 of 55 Photos: Deadly attack at Boston Marathon Women and children are escorted away from the scene. Hide Caption 34 of 55 Photos: Deadly attack at Boston Marathon Bomb squad officials check a possible suspicious device near the scene of the blasts. Hide Caption 35 of 55 Photos: Deadly attack at Boston Marathon A woman is comforted after the blasts. Hide Caption 36 of 55 Photos: Deadly attack at Boston Marathon Two injured women are taken away on stretchers. Hide Caption 37 of 55 Photos: Deadly attack at Boston Marathon An injured woman is loaded into an ambulance. Hide Caption 38 of 55 Photos: Deadly attack at Boston Marathon First responders load injured people into an ambulance. Hide Caption 39 of 55 Photos: Deadly attack at Boston Marathon An injured man is prepared to be moved from a stretcher to an ambulance. Hide Caption 40 of 55 Photos: Deadly attack at Boston Marathon An injured woman is placed on a stretcher. Hide Caption 41 of 55 Photos: Deadly attack at Boston Marathon A runner is comforted following the attack. Hide Caption 42 of 55 Photos: Deadly attack at Boston Marathon The Cambridge Police Department's bomb squad investigates unattended personal items left behind after the explosions. Hide Caption 43 of 55 Photos: Deadly attack at Boston Marathon Bystanders check their mobile devices for news of the explosions. Hide Caption 44 of 55 Photos: Deadly attack at Boston Marathon A member of the bomb squad investigates a suspicious item on the road near Kenmore Square. Hide Caption 45 of 55 Photos: Deadly attack at Boston Marathon A runner in a wheelchair is taken from a triage tent after the explosions went off. Hide Caption 46 of 55 Photos: Deadly attack at Boston Marathon People comfort each near the site of the blasts. Hide Caption 47 of 55 Photos: Deadly attack at Boston Marathon Racers and race officials stand by after the explosions. Hide Caption 48 of 55 Photos: Deadly attack at Boston Marathon Emergency personnel respond to the scene. Hide Caption 49 of 55 Photos: Deadly attack at Boston Marathon Police and emergency crews tend to victims. Hide Caption 50 of 55 Photos: Deadly attack at Boston Marathon An injured woman is carried away on a stretcher. Hide Caption 51 of 55 Photos: Deadly attack at Boston Marathon A man lies on the ground after the incident. Hide Caption 52 of 55 Photos: Deadly attack at Boston Marathon Officials watch as the first explosion goes off on Boylston Street. Hide Caption 53 of 55 Photos: Deadly attack at Boston Marathon Spectators leave the bleachers after the explosions. Hide Caption 54 of 55 Photos: Deadly attack at Boston Marathon Police inspect one of the blast sites. Hide Caption 55 of 55

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."

Photos: Nation mourns Boston bombing victims Photos: Nation mourns Boston bombing victims Three people were killed when two homemade explosives went off at the Boston Marathon on April 15, 2013, and a campus police officer at the Massachusetts Institute of Technology was fatally shot in the manhunt that followed. From left, the victims were Krystle Campbell, Sean Collier, Lingzi Lu and Martin Richard. Click through the gallery to see how the victims were honored and remembered in the weeks after the terror attack. Hide Caption 1 of 50 Photos: Nation mourns Boston bombing victims People pause at the memorial site in Boston's Copley Square on April 30, 2013. Hide Caption 2 of 50 Photos: Nation mourns Boston bombing victims Running shoes were among the mementos left as a tribute to the bombing victims. Hide Caption 3 of 50 Photos: Nation mourns Boston bombing victims Boston showed its resilience and heart with signs of support for the bombing victims, including this cover from an issue of Boston magazine. Hide Caption 4 of 50 Photos: Nation mourns Boston bombing victims Law enforcement officials enter St. Patrick's Church prior to Collier's funeral in Stoneham, Massachusetts, on April 23, 2013. Hide Caption 5 of 50 Photos: Nation mourns Boston bombing victims President Barack Obama observes a moment of silence in the White House Oval Office on April 22, 2013. Hide Caption 6 of 50 Photos: Nation mourns Boston bombing victims Staff members of the Beth Israel Deaconess Medical Center gather inside a trauma room to observe a moment of silence on April 22, 2013. Hide Caption 7 of 50 Photos: Nation mourns Boston bombing victims A Buddhist sits at a memorial near the marathon finish line during a moment of silence on April 22, 2013. Hide Caption 8 of 50 Photos: Nation mourns Boston bombing victims One week after the bombings, people gather to observe a moment of silence in Copley Square. Hide Caption 9 of 50 Photos: Nation mourns Boston bombing victims Officials line Boylston Street as they observe a moment of silence near the marathon finish line on April 22, 2013. Hide Caption 10 of 50 Photos: Nation mourns Boston bombing victims People take part in the moment of silence near the marathon finish line. Hide Caption 11 of 50 Photos: Nation mourns Boston bombing victims State employees pause for a moment of silence on the steps of the Massachusetts State House. Hide Caption 12 of 50 Photos: Nation mourns Boston bombing victims Pallbearers carry Campbell's casket after a funeral service in Medford, Massachusetts, on April 22, 2013. Hide Caption 13 of 50 Photos: Nation mourns Boston bombing victims Mourners walk out of St. Joseph Catholic Church after Campbell's funeral service. Hide Caption 14 of 50 Photos: Nation mourns Boston bombing victims Nurse practitioner Maureen Quaranto, who treated victims of the bombings, wears her Boston Marathon jacket during Mass on April 21, 2013. Hide Caption 15 of 50 Photos: Nation mourns Boston bombing victims From left, Boston Police Department Superintendents Kevin Buckley and William Evans attend Mass with Police Commissioner Edward Davis on April 21, 2013. Hide Caption 16 of 50 Photos: Nation mourns Boston bombing victims Photos of the deceased are displayed in Boston at the Cathedral of the Holy Cross on April 21, 2013. Hide Caption 17 of 50 Photos: Nation mourns Boston bombing victims Leaders of an interfaith service participate in a vigil near the finish line on April 21, 2013. Hide Caption 18 of 50 Photos: Nation mourns Boston bombing victims A makeshift memorial honors the bombing victims on April 21, 2013. Hide Caption 19 of 50 Photos: Nation mourns Boston bombing victims Nicole Collier Lynch, Collier's sister, hugs a police officer during a vigil in Wilmington, Massachusetts, on April 20, 2013. Hide Caption 20 of 50 Photos: Nation mourns Boston bombing victims People gather at a makeshift memorial in Boston on April 20, 2013. Hide Caption 21 of 50 Photos: Nation mourns Boston bombing victims Huntsville, Alabama, residents hold a prayer vigil on April 19, 2013. Hide Caption 22 of 50 Photos: Nation mourns Boston bombing victims A man rings a bell at the vigil in Huntsville. Hide Caption 23 of 50 Photos: Nation mourns Boston bombing victims Two women embrace during a candlelight vigil in Somerville, Massachusetts, on April 18, 2013. Hide Caption 24 of 50 Photos: Nation mourns Boston bombing victims Obama talks with staff members at Massachusetts General Hospital while visiting injured patients on April 18, 2013. Hide Caption 25 of 50 Photos: Nation mourns Boston bombing victims A man at a Boston restaurant watches Obama speak on television on April 18, 2013. Hide Caption 26 of 50 Photos: Nation mourns Boston bombing victims Mourners fill a Boston cathedral for an interfaith service on April 18, 2013. Hide Caption 27 of 50 Photos: Nation mourns Boston bombing victims Obama, first lady Michelle Obama and Massachusetts Gov. Deval Patrick attend the interfaith prayer service. Hide Caption 28 of 50 Photos: Nation mourns Boston bombing victims Flowers, running shoes and other items are left in memory of bombing victim Lingzi Lu. Hide Caption 29 of 50 Photos: Nation mourns Boston bombing victims Dennis Seidenberg of the Boston Bruins observes a moment of silence before the start of an NHL hockey game in Boston on April 17, 2013. It was the first sporting event held in the city after the bombings. Hide Caption 30 of 50 Photos: Nation mourns Boston bombing victims Danielle Cerroni writes a chalk message on the street near the marathon's finish line on April 17, 2013. Hide Caption 31 of 50 Photos: Nation mourns Boston bombing victims A woman looks at memorials left at the scene of the attack. Hide Caption 32 of 50 Photos: Nation mourns Boston bombing victims A crowd gathers at Boston's Garvey Park during a vigil for bombing victim Martin Richard on April 16, 2013. Hide Caption 33 of 50 Photos: Nation mourns Boston bombing victims A woman uses her hand to keep wind from her candle during an interfaith service in Boston on April 16, 2013. Hide Caption 34 of 50 Photos: Nation mourns Boston bombing victims Mourners gather on the edge of the pond for a candlelight vigil in Boston on April 16, 2013. Hide Caption 35 of 50 Photos: Nation mourns Boston bombing victims Students from the Clifden Academy hold an American flag and candles during a vigil in Dorcester, Massachusetts, on April 16, 2013. Hide Caption 36 of 50 Photos: Nation mourns Boston bombing victims People walk along the barricade at Boylston Street on April 16, 2013. Hide Caption 37 of 50 Photos: Nation mourns Boston bombing victims Mourners in Boston hug one another during a vigil for victims on April 16, 2013. Hide Caption 38 of 50 Photos: Nation mourns Boston bombing victims Five-time Boston Marathon runner Jose Sotolongo, center, reacts during a moment of silence in Miami on April 16, 2013. Hide Caption 39 of 50 Photos: Nation mourns Boston bombing victims The New York Islanders and Florida Panthers stand for a moment of silence before an NHL hocky game in Uniondale, New York, on April 16, 2013. Hide Caption 40 of 50 Photos: Nation mourns Boston bombing victims Members of the Boston Red Sox observe a moment of silence before their Major League Baseball game in Cleveland on April 16, 2013. Hide Caption 41 of 50 Photos: Nation mourns Boston bombing victims From left, Boston Marathon runners Tammy Snyder, Diane Deigmann and Lisa Kresky-Griffin embrace at the barricaded entrance to Boylston Street on April 16, 2013. Hide Caption 42 of 50 Photos: Nation mourns Boston bombing victims Two young girls leave flowers on the steps outside the Boston home of 8-year-old bombing victim Martin Richard on April 16, 2013. Hide Caption 43 of 50 Photos: Nation mourns Boston bombing victims Traders at the New York Stock Exchange observe a moment of silence before the opening bell on April 16, 2013. Hide Caption 44 of 50 Photos: Nation mourns Boston bombing victims The flag above the White House flies at half-staff on April 16, 2013. Hide Caption 45 of 50 Photos: Nation mourns Boston bombing victims The Minnesota Twins stand during the national anthem before a baseball game in Minneapolis on April 15, 2013. Hide Caption 46 of 50 Photos: Nation mourns Boston bombing victims Obama makes a statement about the bombings on April 15, 2013. Hide Caption 47 of 50 Photos: Nation mourns Boston bombing victims Players and fans observe a moment of silence before an NBA game in Oakland, California, on Aprl 15, 2013. Hide Caption 48 of 50 Photos: Nation mourns Boston bombing victims An American flag flies at half-staff at the Capitol building in Washington on April 15, 2013. Hide Caption 49 of 50 Photos: Nation mourns Boston bombing victims Hockey fans in Glendale, Arizona, pause for a moment of silence before a game on April 15, 2013. Hide Caption 50 of 50

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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."

Third victim identified

Boston University identified graduate student Lingzi 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. They have 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 bomb maker or bomb makers sought to maximize the suffering.

Dr. George Velmahos, head of trauma care at Massachusetts General Hospital, said his team found "numerous" metal pellets and nails inside patients' bodies.

"There are people who have 10, 20, 30, 40 of them in their body, or more," Velmahos 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.

Victims continue recovery

As investigators continued to search for a suspect, those wounded in the incident continued to recover.

Boston-area hospitals had released at least 112 of the 178 people injured in the attack, according to CNN's tally late Wednesday. Of the 66 people who were still hospitalized, 13 were in critical condition.

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.

Spectator Steve Byrne was standing with a group of friends near a mailbox when the second blast went off. Now his face is scarred with shrapnel wounds. A BB pellet remains lodged in his neck. Doctors said they couldn't remove it because was too close to nerves that control his vision, but he was out of the hospital on Wednesday.

Compared to how his friends are suffering, he told CNN's AC360 that he feels lucky.

Four out of the five friends he was watching the marathon with have lost limbs, he said. One friend had 70 nails in his leg.

He remembers the explosion in vivid detail: the blast that was so strong that it burned his clothes off; the carnage around him; and the haunting, slow-motion daze of searching for his friends.

"We were having a great day and waiting to see our friend cross the finish line," he said, "and then all of the sudden it turned in a flash."

He told CNN he's worried about the financial burdens his friends could face as a result of the explosion.

One friend is a carpenter and "both his hands are incinerated. He can't go to work, and the bills keep coming in."

"It's not just the hospitals. It's everyday life that doesn't stop. ... We're just hoping the mayor, the governor, President Obama don't let us as citizens down."

Beyond those seriously wounded, the incident affected thousands, including Candace Rispoli, who was cheering on a friend when the festive atmosphere turned into a "terrifying hell." She suffered minor injuries.