This image shows the remains of a pressure cooker that the FBI says was part of one of the bombs that exploded during the Boston Marathon. The FBI says it has evidence that indicates one of the bombs was contained in a pressure cooker with nails and ball bearings, and it was hidden in a backpack.
(Photo: FBI via AP)
An FBI tech on Boylston Street looks at evidence on Wednesday in Boston.
(Photo: Robert Deutsch, USA TODAY)
BOSTON (USA TODAY) -- Investigators reported making major progress in the Boston Marathon bombing case Wednesday, including the discovery of an image of a man believed involved in one of Monday's twin explosions that killed three people and injured 176.
A federal law enforcement official told USA TODAY the image is of someone "who we think is involved" in the second bombing. Investigators are enhancing and studying the image, but have not yet determined the man's identity, the official said.
A law enforcement official told USA TODAY that authorities have been focusing on a mass of photographic evidence provided by the public and area security cameras, including an image captured by surveillance cameras of a person placing a bag near one of the bomb sites.
The bag appears similar to the type of black nylon pack described earlier possibly used to carry the explosive devices to the sites, according to officials who have been briefed on the inquiry.
The new details about potential suspects came amid a flurry of earlier news reports that authorities had arrested a suspect, which were later disputed by the FBI.
"Contrary to widespread reporting, there have been no arrests made in connection with the Boston Marathon attack," the FBI said in a statement.
The Associated Press and CNN were among news organizations reporting an arrest. As media reports spread, Boston's federal courthouse was jammed with a massive police and media presence and scores of spectators. The parking lot filled with people with cellphone cameras poised to snap a picture of any suspect.
The courthouse was later evacuated because of a bomb threat, although workers were returning late Wednesday afternoon after authorities conducted a security sweep.
Meanwhile, as investigators painstakingly gather fragments of evidence from the bomb sites, a lid was recovered from a pressure cooker believed used as one of the explosive devices, a federal law enforcement official said.
An official who had been briefed on the matter but was not authorized to comment publicly told USA TODAY the lid was found on a roof near the blast scene.
The discovery came Wednesday as the head of the Department of Homeland Security told a Senate panel in Washington that the Coast Guard worked with the Boston Police Department after Monday's bombings to guard against any potential water-borne attack from Boston Harbor or the Charles River.
Homeland Security Secretary Janet Napolitano said officials continue to investigate the bombing with the FBI as a solitary act of terror.
"There is no current indication to suggest the attack was indicative of a broader plot," Napolitano told the Senate Homeland Security and Governmental Affairs Committee. "But out of an abundance of caution, we continue to keep in place enhanced security measures, both seen and unseen."
At least 14 of those injured in the blast remained in critical condition Wednesday at several area hospitals. Peter Burke, chief trauma surgeon at Boston Medical Center, said two of the 19 patients there still being treated remain in critical condition, including a 5-year-old boy. All, however, are expected to survive, he said.
Burke said patients who required amputations or who lost limbs are now entering the second phase of their recovery, which is making sure that infection does not set in. "They get injured very quickly, but it takes a long time for people to get better," he said.
Evidence investigators from ATF, FBI and other federal agencies wearing protective suits continued poring over the crime scene Wednesday. Evidence trucks and mobile labs filled Exeter Street, the side street off Boylston closest to the blast sites.
The amount of gunpowder used in the bombings is believed to be a fraction of the overall weight of the devices, estimated to be about 20 pounds each, a law enforcement official said Wednesday.
Much of the weight was attributed to the pressure-cooker container and a mix of shrapnel - BB pellets and nail fragments - that cut a deadly path through the crowds gathered near the race finish line, said the official who is not authorized to speak publicly.
The official said the components of the bomb - common kitchen pressure cookers, wire, batteries and gunpowder - are so widely available that barring the assistance of an informant or a telling photo from the crime scene, it will likely take investigators some time to determine where the materials were obtained and who acquired them.
"This is either quick or it's not,'' the official said, referring to the identification of possible suspects, "and right now it's looking like not.''
At the same time, the official said, bomb technicians will likely be able to reconstruct much of the entire device, from both pieces recovered from the scene and the collective knowledge of investigators who have encountered similar devices in past investigations.
"They are going to be able to figure out how this device was acquired,'' the official said. "Depending on the trade craft involved, they will be able to do it relatively easily.''
Boston FBI chief Richard DesLauriers said the recovered materials were being examined at the FBI's laboratory in Quantico, Va., where the bureau has assembled a clearinghouse of improvised explosive devices recovered from such places as the battlefields of Iraq and Afghanistan and crime scenes around the country. Some evidence will undergo an expedited analysis, FBI spokesman Special Agent Jason Pack said.
The scene of the explosions is strewn with shredded T-shirts, metal fragments and glass shards. Boston Police and National Guard soldiers guard every access point, but from the side streets, spectators have watched the investigators at work.
The ATF's evidence recovery experts have found blast debris on rooftops and embedded in nearby buildings, Acting ATF Special Agent Eugenio Marquez said.
"It gives the scope of the power of the blast," Marquez said.
The latest discoveries came as investigators appealed to the public for videos and photos of the scene in hopes of getting an image of the person or persons who left the explosive devices near the finish line of the marathon.
Authorities have yet to determine the motive for the bombings and are urging anyone with tips to come forward with information.
"The person who did this was someone's friend, co-worker or neighbor," DesLauriers said. "Somebody knows who did this."
No one has claimed responsibility for the explosions and "the range of suspects and motives remain wide open."
Meanwhile, a Chinese newspaper has identified the blast's third fatality as Lu Lingzi, a Chinese national and graduate student Boston University.
The Shenyang Evening News, a state-run Chinese newspaper, said the victim is from northeastern China.
An editor at the newspaper said that Lu's father confirmed his daughter's death when reporters visited the family home, the Associated Press reported.
Lu, who previously studied international trade at Beijing Institute of Technology, was studying statistics at BU, according to her Facebook page and media reports.
The other two victims were Martin Richard, 8, of Dorchester, and Krystle Campbell, 29, of Medford, Mass.
The Chinese Consulate in New York said in a statement Tuesday that another Chinese citizen was wounded and was in stable condition following surgery.
Donna Leinwand Leger, Kevin Johnson, Doug Stanglin and Gary Strauss, USA TODAY. Contributing: Bart Jansen; Associated Press