Investigators in the Boston Marathon case are trying to identify a young man seen on video near the second blast site -- talking on his cellphone before setting down a black bag and dashing away, law enforcement officials told NBC News.
As authorities worked to find more images of the man in the crowd and began examining cellphone records in an effort to put a name to his face, they abruptly canceled an evening briefing on the probe.
Two days after the double bombing killed three people and injured 176 near the race finish line — and on the eve of President Obama's visit to Boston for a prayer service — the investigation appeared to be making progress.
Law enforcement officials said the man was seen from several different angles at the site of the second blast. One official said a Lord & Taylor department store surveillance camera captured the image.
The FBI has distributed to federal law enforcement agencies a surveillance photo of a man wearing a baseball cap at the scene of the Boston Marathon bombing, asking officials if they have any information about his identity, a senior federal law enforcement official who has seen the photo told NBC News late Wednesday.
The official who has seen the photo described it as showing a man about six feet tall wearing "a white or off white baseball cap." The FBI was asking for help identifying the individual, the official said.
Federal law enforcement officials have said they have made "significant" progress in the investigation by identifying potential witnesses or suspects who appear in surveillance photos at the site of the second bomb explosion.
But some officials have cautioned they do not yet know the identity of the individuals they are looking for. Asked tonight if investigators knew the identity of the individual identified in surveillance photos, Massachusetts Gov. Deval Patrick said "no." He declined further comment.
An expected press briefing on Wednesday afternoon was postponed and ultimately canceled, but the FBI and other agencies did shoot down afternoon reports from several media outlets that an arrest had been made.
"Despite reports to the contrary there has not been an arrest in the Marathon attack," Boston police said from an official Twitter account.
The FBI added: "Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting."
In another development, the Boston federal courthouse was evacuated Wednesday afternoon. Employees said they were told that a "code red" was in effect and were ordered to leave. The reason was unclear.
Investigators were reviewing more video and photos in an effort to track the movements of the man with the bag before and after the bombing. Records of cellphone calls made in the area at that time could give them another clue to his identity.
Forensic work at the scene has already helped authorities identify major components of the bombs.
They were housed in metal containers — at least one a kitchen pressure cooker — and studded with metal, including fine nails or brads, to make the devices more lethal.
Sources involved in the investigation said that the pressure-cooker device was effectively a "homemade claymore," a directional explosive that appeared to include a triggering mechanism using a battery pack and a circuit board. Both of those elements were recovered at the scene.
The type of battery pack used typically powers toy cars and trucks, and "tens of thousands" have been sold in just the past year, which would make it difficult to trace, said Benjamin Mull, vice president of the manufacturer, Tenergy.
Doctors said they have pulled fragments as large as 2 inches, including pieces of wood, concrete and plastic, from the bodies of people wounded in the attack.
The injuries have been so severe that surgeons had to operate a second time on some patients, even after amputations, to fight possible infection, said Dr. Peter Burke, the chief of trauma services at Boston Medical Center.
A 5-year-old boy was among the patients still in critical condition at the hospital, Burke said. In all, 62 patients were still at Boston hospitals, including 12 critically injured.
The three people killed in the attack were Lingzi Lu, a Boston University graduate student; 8-year-old Martin Richard of Boston; and 29-year-old Krystle Campbell of Medford, a Boston suburb.
Thursday's interfaith service will be dedicated to those gravely wounded or killed. President Barack Obama and first lady Michelle Obama and former Massachusetts Governor Mitt Romney were planning to attend at Cathedral of the Holy Cross in South Boston.
In another homage to the victims and their wounded city, the Boston Bruins wore "Boston Strong" helmet decals for their game Wednesday night.
The game also featured a stirring rendition of the National Anthem, lead by singer Rene Rancourt.