BOSTON -- The investigation of the Boston Marathon bombing focused Wednesday on a department store video of a man placing a black bag that may have contained a bomb near the finish line, authorities said.
The image of the man in the video could represent a break in the case, in which two bombs exploded within seconds of each other Monday, killing three and injuring 176.
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A source told Newsday that the FBI is using the video along with cellphone signals in the area to try to get the names of one or more people who may have been involved in the bombings. A federal law enforcement source said the man was wearing a baseball cap and jacket.
The emergence of the images two days after the attack came amid fast-moving developments, including numerous reports that an arrest had been made. Those reports led to a statement from the FBI, the lead agency in the investigation, that "Contrary to widespread reporting, no arrest has been made in connection with the Boston Marathon attack."
Soon after reports of an arrest were refuted, the city's federal courthouse and a major hospital were evacuated -- developments that further rattled a city reeling from what President Barack Obama has labeled a terrorist attack.
The video of the man who may have planted the explosives -- two pressure cookers packed with nails, ball bearings and scrap metal -- was shot by a camera mounted on the outside of the Lord & Taylor department store on Boylston Street, across from the finish line.
The lid of one of the pressure cookers was found on a nearby rooftop, indicating the force of the blast, a federal source told Newsday.
City and federal authorities scheduled and canceled news conferences Wednesday as developments unfolded. Meanwhile, Secretary of Homeland Security Janet Napolitano has said the twin bombings were not "indicative of a broader plot." No individual or group has claimed responsibility.
Thursday, Obama and the first lady will arrive in this shaken city to attend an interfaith ceremony in honor of the dead and injured.
Many were critically hurt. Several lost limbs as the shrapnel from the explosions ripped through their flesh. Doctors reported a number of people were crippled by nails.
The bombs, authorities said, set off at 2:50 p.m., were designed to hurt as many people as possible and were placed on the ground in dark-colored duffel bags, authorities said. The video of the man being sought by authorities apparently shows one of those bags being placed near the finish line.
As the news unfolded Wednesday, authorities evacuated the John Joseph Moakley federal courthouse and Brigham and Women's Hospital, sending hundreds of people into the street.
"This is really scary," said Mary Clark-Rice, a Boston native who cried as she ran out of the hospital. "I've never seen Boston on edge like this."
Law enforcement released little information, scheduling and canceling news conferences, as residents hoped for speedy closure in an attack that cast a pall on one of the city's most beloved events.
Eleven patients hurt in the blast remain at Massachusetts General Hospital, down from 31 Monday: four are in critical condition, four are serious and three are considered fair. The hospital conducted four amputations and said all patients are expected to live, officials said. Beth Israel Deaconess Medical Center, which treated 24 patients after the blast, had 13 victims as of midday.
A spokesman from Brigham and Women's Hospital said the facility is treating 11 patients, down from 35. Four remain in critical condition. The 35 original patients ranged in age from 16 to 62. One is now an amputee.
A 5-year-old child remained in critical condition at Boston Medical Center Wednesday morning, an official said; 10 were in serious condition and seven in fair condition.
Bostonians continued to leave flowers, including yellow tulips and white daisies, near the site of the attack on Boylston and Berkeley streets.
"We're not going to let this scare us," said Kristen Fam, who came to the site with her son Jason, 10.
A runner's number -- 24971 -- was taped to a nearby railing, along with messages. One read: "Keep on running, Boston" and another said, simply, "Stay strong Boston."
John Goodpaster, head of the forensic science program at Indiana University and a former chemist with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said if the mixture was indeed unstable, it could be something more easily made.
Such homemade materials are sensitive to heat, shock and sparks, while military and commercial-grade explosives are much more stable, he said.
Special agent Steven Bartholomew, a spokesman for ATF, said Tuesday afternoon the blasts were large and sent objects high over buildings.
"Some debris were on top of rooftops, embedded in buildings," he said.