U.S. authorities have zeroed in on a possible suspect in the deadly bombing at the Boston Marathon.
An image of the man – who has not yet been identified by name – was captured on video footage taken close to the explosions, which killed three people and wounded more than 170 others.
The news of the potential breakthrough came on a jittery day of fresh threats and scares across the U.S. In Washington, a preliminary test showed traces of ricin, a lethal toxin, in a letter addressed to President Barack Obama. And in Oklahoma City, officials briefly evacuated a municipal building for what turned out to be a false bomb scare.
There was no indication any of these scares were connected to the attack in Boston, but they feed into a new sense of fear radiating through the United States days after the bombing.
Late Wednesday, authorities said they had arrested a man in Corinth, Miss., in connection with two letters laced with ricin, intended for Mr. Obama and a Mississippi senator.
Three suspicious packages in U.S. Senate offices sparked an alarm but were deemed harmless.
On Thursday morning, Mr. Obama will travel to Boston to participate in an interfaith service honouring the victims.
The city is growing impatient, longing to put a face to whoever is responsible for Monday's attack. Scores of New Englanders flocked to the seaside federal courthouse after news broke Wednesday – erroneously, it turned out – that a suspect was being brought in for arraignment.
They proved their hunger for justice by standing outside the courthouse even after it was evacuated due to a bomb scare that proved to be just that – a scare. One man, Mike Murphy, had brought his bagpipes to "play patriotic music in [the suspect's] face."
He never got the chance. Instead, Mr. Murphy, like many others, was whipsawed by conflicting media reports about the progress of the investigation.
On Wednesday afternoon, the Associated Press reported that an arrest in the bombing case was imminent. On television, CNN went even further, reporting that a suspect was in custody and U.S. marshals were taking him to court.
A frantic scramble ensued, as hundreds of journalists from the world's media outlets rushed to Boston's federal courthouse.
In less than an hour, however, the earlier reports were proven false. Several law-enforcement agencies issued denials that an arrest had taken place.
The episode prompted an official rebuke from the U.S. Federal Bureau of Investigation. "These stories often have unintended consequences," the agency said in a statement Wednesday afternoon. "We ask the media, particularly at this early stage of the investigation, to exercise caution."
Investigators were focusing on a man they pinpointed in two videos, CNN reported, including one from a surveillance camera at Lord & Taylor, a department store across from the site of the second explosion. The video, the channel said, showed the suspect leaving a bag believed to contain the bomb – a pressure cooker packed with explosives and shrapnel. The man was in a light hooded sweatshirt and on his head was a white baseball cap, worn backward, it said.
Authorities were reportedly making progress in identifying the man, but not fast enough for the crowd that gathered outside Boston's federal courthouse on Wednesday afternoon, drawn by rumours of an arrest.
"We want to put a face to such a horrific event," said South Boston resident Adam Frew as a Coast Guard helicopter circled overhead.
"We all want justice," echoed Doug Crabtree, who was there with his wife and two young children.
By Wednesday, Boston had returned to a semblance of normality, except in the area immediately surrounding the bombing. But even there, several streets previously inaccessible were opened to traffic by the afternoon and sidewalk cafés were full of customers enjoying the sunny weather.