BOSTON — The FBI Thursday released video clips and photos of two nameless "armed and extremely dangerous" suspects in the Boston Marathon bombings and asked for the public's help in identifying them.
"Somebody out there knows these individuals," said Boston FBI Special Agent Richard DesLauriers. "Thought it may be difficult, the nation is counting on those with information to come forward and provide it to us."
The images show two men in baseball caps, one of whom is seen putting down a black backpack at the second blast site. Both were seen walking together through the crowd of spectators.
"We consider them to be armed and extremely dangerous," said DesLauriers. "No one should approach them. If you see these men, contact law enforcement."
Investigators said the suspects images came from a surveillance camera in the finish-line area of the marathon. Investigators have spent hours poring over video, frame by frame, since the bombings Monday at 2:50 p.m. that killed three and injured 175, many severely.
In Washington earlier in the day, Homeland Security Secretary Janet Napolitano told a congressional committee "we need the public's help in locating these individuals." She also said it is too soon to say if the Boston bombings are connected to a domestic or foreign terrorist plot.
"This is not an NCIS episode," she said. "Sometimes you have to take time to properly put the chain together to identiy the perpetrators, but everybody's committed to seeing that that gets done in the right way."
President Obama, who called the bombings an act of terrorism, and his wife, Michelle, attended an interfaith hearling service Thursday at Boston's Cathedral of the Holy Cross. The pews in the front of the church were filled with family and friends of the victims.
"Our prayers are with the injured -- so many wounded, some gravely," Obama said. "From their beds, some are surely watching us gather here today. And if you are, know this: As you begin this long journey of recovery, your city is with you. Your commonwealth is with you. Your country is with you. We will all be with you as you learn to stand and walk and, yes, run again. Of that I have no doubt. You will run again. You will run again.”
Afterwards, the president and first lady visited with the wounded and their doctors and nurses at three Boston hospitals. Obama also met with hundreds of first responders and race volunteers at Cathedral High School, praising them for embodying "the best of the American spirit."
Earlier, the president signed an emergency declaration to release federal funds to help Boston recover from the tragedy. More than 60 of the injured remain hospitalized, including a dozen in critical condition. Doctors reported that 15 children and adults had limbs amputated as a result of severe flesh, bone and blood vessel wounds.
Investigators believe the bombs were hidden in black duffel bags or backpacks, and constructed of common household pressure cookers packed with nails, ball bearings and buckshot-like BBs. They said they were detonated by timers or remote devices, causing metal shrapnel to spray spectators.
U.S. security officials said similar bombs have been used in terrorist attacks in India, Nepal, Pakistan and Afghanistan.
Killed in the Boston bombings from the metal projectiles was 8-year-old Martin Richard of Boston. A widely distributed grade school photo of him holding a handmade poster saying, "No more hurting people. Peace," has become an international symbol of the tragedy.
Also killed in the blasts were Krystle Campbell, 29, a restaurant manager in Arlington, Mass., and Lu Lingz, 23, a Chinese national graduate student at Boston University.
President Obama, in his church remakrs, assured their familes that authorities would find the bombers and make them face justice for their evil deed.
"We will find you," he said. "We will hold you accountable, but, more than that, our fidelity to our way of life, to a free and open society, will only grow stronger, for God has not given us a spirit of fear and timidity but one of power and love and self discipline.”
Details for this story were provided by law enforcement officials investigating the bombing case.