BOSTON, Mass. (CBS News/AP) — In what could be a major break in the Boston Marathon bombings case, investigators are on the hunt for a man seen in a department-store surveillance video dropping off a bag at the site of the bombings.

The development – less than 48 hours after the attack that left three people dead and more than 170 wounded, including an 11-year-old Bay Area boy – marked a possible turning point in a case that has investigators analyzing photos and videos frame by frame for clues to who carried out the twin bombings and why.

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Law enforcement sources told CBS News that investigators were trying to identify a person described as a young white man who is standing in the crowd near the scene of the second bombing just before the device exploded. A surveillance camera at a nearby Lord and Taylor department store captured images of the man who was carrying a backpack and talking on a cell phone.

Boston City Council President Stephen Murphy, who said he was briefed by Boston police, said investigators matched the findings from the image on surveillance footage with witness descriptions of someone leaving the scene.

“I know it’s very active and very fluid right now – that they are on the chase,” Murphy told The Associated Press. He added: “They may be on the verge of arresting someone, and that’s good.”

The man was wearing a black jacket, a grey hooded sweatshirt and a white or off-white baseball cap backwards. He was described as 6 feet or 6-feet-2-inches tall with a medium build.

Murphy said the department store video “has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off” when the first explosion occurred at the finish line about 100 yards down Boylston Street. Just a few seconds later, the second bomb exploded near where the man had been standing.

Investigators now are going through cell phone logs to determine who made calls from that location near the time of the explosions. Sources told CBS News that the FBI was working with a list of names of cell phone owners and attempting to match one of those to the unknown man on the surveillance

The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, investigators and others close to the case said. Investigators suspect the devices were then hidden in black duffel bags and left on the ground.

As a result, they were looking for images of someone lugging a dark, heavy bag.

Law enforcement sources who spoke to both CBS News and the AP confirmed that investigators had an image of a potential suspect whose name was not known to them and who had not been questioned.

The turn of events came with Boston in a state of high excitement over conflicting reports of a breakthrough.

Law enforcement officials briefed on the investigation told both the AP and CNN around midday that a suspect was in custody and was expected in federal court. But the FBI and the U.S. attorney’s office in Boston said no arrests had been made.

By nightfall, there was no evidence anyone was in custody. No one was brought to court. The law enforcement official, who had affirmed to the AP that there was a suspect in custody even after federal officials denied it, was unable to obtain any further information or explanation.

At least 14 patients, including 11-year-old Bay Area victim Aaron Hern of Martinez, remained in critical condition. Dozens of other patients have been released from hospitals around the Boston area, and officials at three hospitals that treated some of the most seriously injured said they expect all their remaining patients to survive.

On Wednesday, investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues. They picked through trash cans, plastic cup sleeves and discarded sports drink dispensers

Boston remained under a heavy security presence, and some people admitted they were nervous about moving about in public spaces.

Police were stationed on street corners across downtown Boston, while National Guardsmen set up tents on the Boston Common and stationed tactical vehicles.

President Barack Obama planned to visit Boston on Thursday to attend a service honoring the victims.

The blasts killed 8-year-old Martin Richard of Boston and 29-year-old Krystle Campbell of Medford, Mass. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi, a graduate student at Boston University.

Heightening jitters in Washington, D.C., where security already had been tightened after the Boston bombings, the FBI arrested Mississippi man on Wednesday, accused of sending letters to Obama and a senator that tested positive for the poisonous ricin.

The man was identified as Paul Kevin Curtis, 45, of Corinth, near the Tennessee state line about 100 miles east of Memphis. There was no indication the mailings were related to the Boston attack.

(Copyright 2013 CBS San Francisco, CBS Interactive and the Associated Press. All rights reserved.)


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