BOSTON -- Investigators reported making major progress in the Boston Marathon bombing case Wednesday, including locating an image of a man believed involved in one of the explosions.
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, who described the investigation as "very active,'' told USA TODAY that authorities have been focusing on a mass of photographic evidence provided by the public and area security cameras. But the source indicated that there mixed views on the value of the analysis so far.
"There is a lot going on," said the official, who is not authorized to comment publicly on the investigation.
CNN's John King quoted law enforcement sources as saying that surveillance video from a nearby Lord &Taylor department store was key in the investigation. Television footage also played a role, CNN reports.
"The camera from Lord & Taylor is the best source of video so far," said Dot Joyce, a spokeswoman for Boston Mayor Thomas M. Menino told the Boston Globe. "All I know is that they are making progress."
The new details about potential suspects came amid a flurry of earlier news reports that authorities had arrested a suspect, which were later dismissed 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. People with cell phone cameras poised to snap a picture of any suspect filled the parking lot.
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 two explosions that killed three people and injured 176 Monday afternoon, 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 has been briefed on the matter but is not authorized to comment publicly told USA TODAY the lid was found on a roof near the blast scene.
The discovery came 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.
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 at the bomb scene 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.
At the blast site, evidence investigators from ATF, FBI and other federal agencies wearing protective suits continued poring over the crime scene Wednesday. Evidence trucks and mobile labs fill Exeter Street, the side street off Boylston closest to the blast scenes.
The amount of gunpowder used in the Boston Marathon 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 IED devices recovered from places ranging from the battlefields of Iraq and Afghanistan to crime scenes around the country.
The scene 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.
Some evidence is being flown to the FBI lab and will undergo an expedited analysis, FBI spokesman Special Agent Jason Pack said.
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, however, 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 atrocity and "the range of suspects and motives remain wide open."
Meanwhile, a Chinese newspaper has identified the third victim of Monday's deadly blast 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 Boston, 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.