BOSTON (AP) - Investigators were on the hunt Thursday for a man seen in a department-store surveillance video dropping off a bag at the site of the Boston Marathon bombings that killed three people, a Boston politician said.
In what could be a major break in the case, a law enforcement official separately confirmed that authorities have found an image of a potential suspect but don't know his name.
Meanwhile, the third victim was identified as Boston University graduate student Lu Lingzi by The Shenyang Evening News, a state-run Chinese newspaper. The blasts also killed 8-year-old Martin Richard of Boston and 29-year-old Krystle Campbell of Massachusetts.
Wednesday's developments -- less than 48 hours after the attack that also wounded more than 170 at the world's oldest and most prestigious marathon -- 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. The footage hasn't been made public.
City Council President Stephen Murphy, who said he was briefed by Boston police, said investigators saw the image on surveillance footage they got from a department store near the finish line, and matched the findings 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 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.
One department store video "has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off," Murphy said.
A law enforcement official who spoke to AP on condition of anonymity and was not authorized to discuss the case publicly confirmed only 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.
A law enforcement official briefed on the investigation told the AP around midday that a suspect was in custody. The official, who was not authorized to divulge details of the investigation and spoke on condition of anonymity, said the suspect 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 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 remained in critical condition. Dozens of 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
President Barack Obama is planning to attend a service honoring the victims Thursday in Boston, where police were stationed on street corners across downtown and some residents admitted they were nervous moving about in public spaces.
People lined up hours ahead of time Thursday morning to get into the city's Roman Catholic cathedral for the interfaith service.
Dr. Horacio Hojman, associate chief of trauma at Tufts Medical Center, said patients were in surprisingly good spirits when they were brought in.
"Despite what they witnessed, despite what they suffered, despite many of them having life-threatening injuries, their spirits were not broken," he said. "And I think that should probably be the message for all of us -- that this horrible act of terror will not bring us down."
Compounding Americans' jitters were letters sent to Washington officials, including ones addressed to Obama and a senator, that contained suspicious substances that showed traces of poisonous ricin in initial tests. The letters evoked eerie parallels to the anthrax attacks that followed the terrorist attacks of Sept. 11, 2001.
It's against that backdrop that Obama, along with his wife, Michelle, heads to Boston for the "Healing Our City" service. He may also meet with some of those injured, as well as the first responders who rushed toward the blast to help the scores of runners and spectators.
"We send our support and encouragement to people who never expected that they'd need it -- the wounded civilians
who are just beginning what will be, I'm sure for some of them, a long road to recovery," Obama said Wednesday in a likely preview of his remarks at the service.
The president has stepped into this role as America's consoler in chief many times before in his presidency, most recently in December after the massacre of 20 first-graders and six educators at an elementary school in Newtown, Connecticut. Before that, there were the deadly shootings in Aurora, Colorado, Tucson, Arizona, and Fort Hood, Texas, as well as the natural disasters like Superstorm Sandy, which decimated the East Coast last year.
This time, Obama must confront the unique challenges of a terror attack that inevitably revived memories of Sept. 2001. As he did in a statement from the White House on Tuesday, the president was expected to urge the public to remain vigilant, while declaring that "the American people refuse to be terrorized."
Obama signed an emergency declaration for Massachusetts on Wednesday and ordered federal aid to supplement the local response to the bombings.
Associated Press writers Jay Lindsay, Pat Eaton-Robb, Steve LeBlanc, Bridget Murphy and Meghan Barr in Boston; Eileen Sullivan, Julie Pace and Lara Jakes in Washington; and Marilynn Marchione in Milwaukee contributed to this report.