The FBI on Thursday released photographs and video of two men said to be suspects in the Boston Marathon bombings that killed three spectators and injured more than 170 other people.

Richard DesLauriers, special agent in charge of the FBI office in Boston, appealed to the public for help in identifying the two men, whom he cautioned should be considered “armed and extremely dangerous.”

Appearing at a news briefing with U.S. Attorney Carmen Ortiz, DesLauriers said the two men, both carrying what appeared to be heavy bags on their backs, walked together through the crowd of spectators. He said the man identified as Suspect No. 2, wearing a white cap, was seen leaving his bag at the site of the second explosion Monday. The other man, Suspect No. 1, wore a dark cap.

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Surveillance video released by the FBI Thursday shows the two men they have identified as suspects in the Boston Marathon bombing. (Courtesy of FBI)

“We initially developed a single person of interest,” not knowing whether the man was acting alone or with others, DesLauriers said. The FBI later determined that there was a second suspect, he said.

“Today we are enlisting the public’s help to identify the two suspects,” he said. Photos of the men were displayed on easels set up in the briefing room, and DesLauriers said the images would also be published on the FBI’s Web site.

“Somebody out there” knows who the men are, DesLauriers said, adding: “We consider them to be armed and extremely dangerous.” He warned the public: “No one should approach them.... Do not take any action on your own.” He urged people instead to contact law enforcement.

Earlier Thursday, Homeland Security Secretary Janet Napolitano told a House committee in Washington that the FBI wants to interview individuals seen in at least one video from the vicinity of the Boston Marathon finish line, but she would not describe them as suspects.

Testifying before the House Homeland Security Committee, Napolitano did not provide details of the men’s appearance or say what the video shows, the Associated Press reported. “There is some video that raised the question” of what the men were doing, she said.

Napolitano said it remains unclear whether the bombs that exploded Monday near the finish line of the renowned race were the work of foreign or domestic terrorists.

“We don’t know yet whether the attack was planned and executed by a terrorist organization, foreign or domestic, or if it was an individual act,” James R. Clapper Jr., the director of national intelligence, told lawmakers during a separate hearing on worldwide threats. “Lone wolves, domestic extremists and jihad-inspired or -affiliated groups are certainly determined to attack,” he added in testimony before the Senate Armed Services Committee.

With the investigation proceeding, President Obama, first lady Michelle Obama and members of the Massachusetts congressional delegation flew to Boston to attend an interfaith prayer service, console victims of the bombings and their relatives and thank medical personnel and first responders.

Napolitano spoke a day after the daunting task of sifting through thousands of images of the Boston Marathon bombing site in search of a culprit suddenly telescoped to a video from a Lord & Taylor security camera.

The discovery of video of a man who wore a large backpack to the finish line area and then dropped the package there raised hopes Wednesday for an imminent breakthrough in the case, setting off a media frenzy and insistent statements from authorities that no arrest has been made. A Boston city official said the video is of “special interest” to investigators.

The second full day of the investigation into the attack brought jitters, rumors and at least the hope that investigators had made important progress. Massachusetts Gov. Deval L. Patrick (D) said that while the probe is “making some progress . . . it’s going to be slow, it’s going to be methodical.”

Boston’s federal courthouse, where hundreds had gathered in response to false reports of an arrest, was briefly evacuated Wednesday because of a bomb threat. Officials also evacuated a Boston hospital, Brigham and Women’s, and Oklahoma City’s City Hall because of suspicious vehicles outside. No explosives were found in those cases.

In Boston, many of the injured were released from hospitals. At Brigham and Women’s, which initially treated 35 people, only 11 were still hospitalized Wednesday evening, four of them in critical condition.

And Boston University released the name of a previously unidentified woman who was killed in the blast. Lu Lingzi, a graduate student in math and statistics at the university and a Chinese national, was watching the race with friends when the bombs blew up.

Wednesday’s whirlpool of reports demonstrated the extraordinary promise and power that new technologies bring to criminal investigations, but also the risk and unreasonable expectations that now permeate such probes. When federal authorities asked the public for help Monday, they received thousands of video clips and still images of the bomb site.

Some people, empowered by smartphones and ever more sophisticated technology, didn’t leave the detective work to the professionals. They joined forces on sites such as to examine crowd pictures, searching for — and then virally distributing — image of backpacks that resembled the shredded bag in photos the FBI released Tuesday.

Black backpacks turn out to be ubiquitous, and when five of them were found in a single photo of the crowd on Boylston Street, the search quickly drew criticism from readers worried that innocent people could be harmed by being identified as suspicious.

Others questioned whether black backpacks were even the most important lead, recalling the search for white box trucks that steered investigators astray in the D.C. sniper case a decade ago.

At midday Wednesday, news organizations such as CNN and the Associated Press reported that investigators had identified a suspect or made an arrest, leading the FBI to issue an unusual appeal: “Contrary to widespread reporting, there have been no arrests made in connection with the Boston Marathon attack. . . . Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting.”

CNN said it “had three credible sources on both local and federal levels” for its reports. “As soon as our sources came to us with new information, we adjusted our reporting.”

In Boston, police barricades still ringed a 12-block area around the finish line, guarded by city police and the Massachusetts National Guard.

Doug Silkwood, a runner wearing his official marathon jacket, stopped at the barricade and recalled how he’d expected his first Boston Marathon to end with “hundreds of thousands of people having a great time.” The bombings dashed all that, said Silkwood, 47, an engineer from San Jose: “It’s probably easier to protect a Boston Celtics game than an open event like the marathon,” but he said he plans to return next year, “to do it out of spite.”

At Boston Medical Center, Jenny Chung, a 35-year-old teacher, was released at midday Wednesday, less than 48 hours after shrapnel was blown into her chest, two inches from her heart.

She wore an honorary marathon finisher’s medal and carried a teddy bear that relatives had sent her. The medal was a gift from a marathon volunteer; Chung did not run the race, but was a spectator, there to watch a friend whose run ended prematurely.

Chung had been poised to video her friend’s triumphant finish when she was knocked to the ground. She felt little pain as she and her friends hurried away.

Only when she got to a friend’s apartment did she see blood oozing from her chest. She went to the hospital, where doctors quickly operated to remove the fragment. Investigators collected all her clothing, including her sneakers, for possible clues, she said.

At the finish line, Chung had stood with her runner friend’s boyfriend, who held a half dozen yellow helium balloons. Shrapnel from the blast cut open the boyfriend’s calf, and popped some of the balloons. The rest slipped from his grasp as he fell. In video of the scene, a few yellow balloons drift upward, above the carnage.

Branigin reported from Washington. Vernon Loeb in Boston, Steven Mufson in Beijing and Sari Horwitz, Ellen Nakashima and Caitlin Dewey in Washington contributed to this report.