After three days of poring over photos and video , investigators appealed to the public to help them identify two men now considered suspects in the Boston Marathon bombings . The men were photographed walking down Boylston Street , one behind the other , near the finish line of Monday 's race . Suspect 1 was seen wearing a light-colored , collarless shirt underneath a dark-colored jacket and wearing a dark baseball cap . The man identified as Suspect 2 was seen setting down a backpack at the site of the second explosion `` within minutes '' of the blasts that killed three people and wounded nearly 180 , said Special Agent Rick DesLauriers , the head of the FBI 's Boston office . He was wearing a light-colored hooded sweatshirt , a black jacket and a white baseball cap turned backward . In particular , DesLauriers asked for help from anyone standing in front of the Forum restaurant , where the second bombing happened . `` Somebody out there knows these individuals as friends , neighbors , co-workers or family members of the suspects , '' DesLauriers said . `` And though it may be difficult , the nation is counting on those with information to come forward and provide it to us . '' People with possible information on the two men were urged to go to the FBI 's website , https : //bostonmarathontips.fbi.gov , or call 800-CALL-FBI ( 800-225-5324 ) . By Thursday evening , authorities already had received a `` large volume of calls ... as a result of the photos , '' an FBI official said . The FBI 's website , moreover , had been inundated with record traffic . It was n't immediately known then whether any of the tips had led to the suspects . DesLauriers cautioned that anyone who think they know their identities should be careful , and consider them armed and `` extremely dangerous . '' `` No one should approach them , '' he said . `` No one should attempt to apprehend them except law enforcement . '' 'They acted differently than everyone else ' Other footage , still unreleased , shows that the two suspects stayed at the scene to watch the carnage unfold , a federal law enforcement official with knowledge of the investigation told CNN 's Susan Candiotti . `` When the bombs blow up , when most people are running away and victims were lying on the ground , the two suspects walk away pretty casually , '' said the official , who has seen the unreleased video . `` They acted differently than everyone else . '' While video of at least one suspect planting the bomb exists , the FBI had chosen not to release it , according to the official . One reason , according to the official , is that were the media to repeatedly show the suspects leaving the bomb , it might cause some people to overreact if they came into contact with them . DesLauriers said intelligence had been developed on the first suspect `` within the last day or so . '' The official who spoke with CNN said images of the second suspect were isolated Wednesday . Flood of digital evidence a blessing and a curse . The investigation also turned toward the possibility that the bombs had been detonated by remote control , a federal law enforcement official briefed on the investigation said Thursday . Investigators contacted the maker of a battery found in the debris of the blasts , said Benjamin Mull , a vice president at Tenergy Corporation . One of the firm 's batteries , typically used in remote-controlled hobby cars , was found in the aftermath of the attack , connected to some wires and a piece of plastic . In the early hours of the investigation , a law enforcement official told CNN that the bombs were probably detonated by timers . The FBI has said details of the detonating system were unknown . While the latest clues moved the investigation forward , it is still unclear whether the attack was an act of domestic or foreign terrorism . Bombers 'picked the wrong city , ' Obama says . Thursday evening 's FBI announcement capped a day in which President Barack Obama brought a mixture of reassurance and defiance to an interfaith memorial service in the city 's Cathedral of the Holy Cross . Whoever planted the bombs `` picked the wrong city '' to attack , he said . `` Every one of us stands with you , '' Obama told the crowd . `` Boston may be your hometown -- but we claim it , too . ... For millions of us , what happened on Monday is personal . '' Addressing the still-unknown perpetrators , Obama added : `` Yes , we will find you . And yes , you will face justice . We will hold you accountable . '' And he looked ahead to next year 's race , predicting that `` the world will return to this great American city to run even harder and to cheer even louder for the 118th Boston Marathon . Bet on it . '' How we 're affected as a nation by the bombing , other incidents . Among the crowd of about 2,000 were first lady Michelle Obama ; the president 's Republican challenger last November , former Massachusetts Gov . Mitt Romney ; the state 's current governor , Deval Patrick ; and Boston Mayor Thomas Menino . During an interlude , attendees were soothed by a performance by famed cellist Yo-Yo Ma . Menino praised each of the three bystanders who were killed in the blasts -- Martin Richard , described as a `` young boy with a big heart '' ; Krystle Campbell , whose spirit `` brought her to the marathon year after year '' ; and Lingzu Lu , who `` came to the city in search of an education . '' The audience also included scores of police officers and other first responders . Crowds erupted in cheers as the cathedral emptied out at the end of the service , while others sang the national anthem . How to help . Obama also stopped at a high school to thank a group of first responders and volunteers and met with patients recovering from the attacks at Massachusetts General Hospital . `` It was extremely uplifting for them , '' said Dr. David King , a trauma surgeon at the hospital . `` I think it 's incredibly inspiring that he would take time out and visit them and have a sincere interaction with all of these folks that have been hurt . '' Obama `` was humbled by the patients ' bravery and their fortitude and their drive to continue , '' he added . The first lady met with patients , families and hospital staff at Boston Children 's Hospital and Brigham and Women 's Hospital , the White House said . The wounded : 59 still hospitalized . Investigators say the bombs , which exploded 12 seconds apart , were designed to deliver the most vicious suffering . A total of 59 people remained hospitalized on Thursday , six of them in critical condition , according to Boston-area hospitals . At least 13 people lost limbs as a result of the bombing . One device was housed in a pressure cooker hidden inside a backpack , the FBI said . The device also had fragments that may have included nails , BBs and ball bearings , the agency said . The second bomb was in a metal container , but it was unclear whether it was in a pressure cooker as well , the FBI said . 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 . They were sent to the FBI 's national laboratory in Virginia , where technicians will try to reconstruct the devices . In the past , the U.S. government has warned federal agencies that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel and detonating them with blasting caps . Family and friends , meanwhile , were mourning the three dead : . • Richard , the 8-year-old boy with a gap-tooth grin and bright eyes . He loved to run and play in his yard . • Campbell , a 29-year-old freckle-faced woman described by her mother as having `` a heart of gold . '' • Lu , the Chinese graduate student at Boston University who had moved to the city last fall , making friends and soaking up new experiences . The U.S. State Department has been in contact with her family and the Chinese Foreign Ministry , Secretary of State John Kerry said in a statement released Thursday . `` We stand ready to provide whatever appropriate assistance we can to the family members of foreign nationals in the aftermath of this despicable act of terror , '' Kerry said .
The FBI gets a `` large volume '' of calls after releasing photos of suspects , official says . `` Somebody out there knows '' the 2 suspects in the case , an FBI agent says . The 2 men walked away from the scene `` pretty casually , '' a federal official says . At an interfaith service , Obama tells Boston attackers : `` We will find you ''