Donna Leinwand Leger, Kevin Johnson and Doug Stanglin, USA TODAY
Investigators reported major progress in the Boston Marathon bombing case Wednesday, but that there had been no arrest, despite conflicting news accounts.
Two law enforcement sources told USA TODAY no arrest had been made. The Associated Press, CNN and Fox Boston said that a suspect had been taken into custody in early afternoon.
PHOTOS: Boston Marathon bombings
Outside the federal courthouse, there is a large law enforcement presence. Media are cramming the sidewalk. People with cell phone cameras poised also jammed the parking lot.
The Associated Press reported Wednesday that a suspect is in custody in the bombing case and was expected to appear in federal court.
The AP report came as several news sources said investigators had identified images of a suspect seen delivering one of the explosive devices near the marathon finish line.
CNN's John King quotes the 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 tells the Boston Globe. "All I know is that they are making progress."
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.
As investigators painstakingly gather fragments of evidence from the two explosions that killed three people and injured more than 170, a lid has been recovered from a pressure cooker apparently used as the explosive device, a federal law enforcement official said.
The 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 scene of the blast.
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 the bombings Monday to guard against any potential water-borne attack from the 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 15 of the 170-plus people 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 were 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.
Richard DesLauriers, the FBI special agent of the Boston office, confirmed on Tuesday that investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker.
Authorities, however, have yet to determine the motive for the bombings or who was behind them, and are urging anyone with tips to come forward with information.
"The person who did this was someone's friend, co-worker or neighbor," he said. "Somebody knows who did this."
No one has claimed responsibility for the atrocity and "the range of suspects and motives remain wide open," DesLauriers said.
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, in a post on its official Twitter-like microblog account, identified the victim, who is from the northeastern Chinese city, the Associated Press reports.
An editor at the newspaper said that Lu's father confirmed his daughter's death when reporters visited the family home, the AP 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.
Contributing: Bart Jansen; Associated Press