Investigators claim major progress in Boston bombings

• Images of suspect in nearby department store being studied
• Janet Napolitano says officials continue to investigate the bombing with the FBI as a solitary act of terror
• Bomb technicians will likely be able to reconstruct much of the device

Investigators are exploring a number of promising leads in the Boston Marathon bombing case, including an image of a man believed possibly involved in one of Monday's twin explosions that killed three people and injured 176.

A federal law enforcement official told USA TODAY Wednesday that the image, captured by surveillance cameras in the area of one of the bomb sites, shows a man putting down a bag near one of the bomb sites. The bag appears to be similar to the black nylon pack described earlier as possibly used to carry the explosive devices, officials who have been briefed on the inquiry told USA TODAY.

Authorities were working to identify the individual in surveillance video obtained from a nearby Lord & Taylor department store, as well as a number of other people in video and photographs near the site of the explosions. More than 70 people remain hospitalized, with 14 listed in critical condition.

The leads emerged during a chaotic day in which federal authorities dismissed a flurry of media reports, including dispatches from the Associated Press and CNN broadcasts, that a suspect had been taken into custody.

"Contrary to widespread reporting, there have been no arrests made in connection with the Boston Marathon attack,'' the FBI said in a statement.

The events in Boston included an evacuation at the federal courthouse sending streams of people into the street just two days after the explosions ripped through crowds near the finish line of Monday's iconic sporting event. Courthouse employees returned late Wednesday afternoon after authorities conducted a security sweep.

The FBI also announced the arrest of Paul Kevin Curtis of Corinth, Miss., in connection with a separate inquiry into a series of suspicious letters addressed to federal lawmakers and President Obama. Preliminarily tests had indicated the letters tested positive for the ricin; the mail was undergoing additional examination for more conclusive testing.

Federal law enforcement officials don't believe the two investigations — the tainted mail and the Boston bombings — are related.

Meanwhile, as investigators painstakingly gathered 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 on Wednsesday. "But out of an abundance of caution, we continue to keep in place enhanced security measures, both seen and unseen."

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 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 to scour 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 weigh 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."

Officials at a Northern California battery company said they believe a battery they manufacture was used in the Boston Marathon bombing.

Benjamin Mull, spokesman for Fremont-based Tenergy Corp., said that based on crime scene photos that have appeared online, the company believes one of its nickel-metal hydride batteries was used to make the bombs that detonated Monday near the marathon's finish line.

"We're all horrified and appalled that our off-the-shelf product would be used in such a horrific way," Mull said.

The company has reached out to Boston police and the FBI but hasn't heard directly from investigators, Mull said.

The company says the 1.25-volt battery seen in the photos is sold in retail outlets and is frequently used by hobbyists for various toys, including radio-controlled cars and trucks.

Tenergy has been selling the battery for several years and said it has sold "tens of thousands" of the particular battery in the past year.

Meanwhile, the blast's third fatality as Lu Lingzi, a Chinese national and graduate student Boston University. 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.

Contributing: Bart Jansen; Associated Press