Investigators have spotted a Boston Marathon bombing suspect from security video taken before two blasts ripped through central Boston on Monday, a US law enforcement source has said, in what is potentially the biggest break in the case yet.
No arrests had been made, and the suspect in the video had not been identified by name, two US government officials said on Wednesday.
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Police considered making an appeal to the public for more information at a news conference, a US government source said, but the FBI cancelled that news conference after delaying it several times, Boston police said.
The bombings, as well as subsequent reports that someone tried to mail a suspected ricin letter to US President Barack Obama - the second report of such a letter in two days - have created a climate of uncertainty in the country.
Earlier the FBI had said there was no indication of a connection between the ricin letters and the Boston bomb attacks.
The federal courthouse in Boston was evacuated amid reports a suspect for Monday's bombings at the Boston Marathon was due to appear in court, but officials said they cleared the area because of a bomb scare.
Al Jazeera's Alan Fisher, reporting from Boston, said reports of an arrest made had been debunked by the FBI and other Boston bombings investigators.
Second bombing scene
He said that US law enforcement sources had identified a suspect in the marathon bombing from surveillance video from a department store near the explosion.
"They have an image that was caught on tape from the second bombing scene," Fisher said.
They have an image that was caught on tape from the second bombing scene. You can see clearly what the man was wearing.
"You can see clearly what the man was wearing."
Our correspondent said the FBI was expected to elaborate more at a news conference. He said the briefing had been delayed because of the bomb scare.
Authorities have an image of a suspect carrying a black bag at the second bomb site.
Police may make an appeal to the public for more information, a US government source said.
Investigators of the bombings searched thousands of pieces of evidence from cell phone pictures to shrapnel shards pulled from victims' legs.
Based on shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have made bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race watched by crowds of spectators.
The blasts at the finish line of Monday's race injured 176 people and killed three: an 8-year old boy, Martin Richard, a 29-year-old woman, Krystle Campbell and a Boston University graduate student who was a Chinese citizen.
Boston University identified the student as Lu Lingzi.
No one has claimed responsibility for the attack.