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 said on Wednesday, 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. Police may make an appeal to the public for more information at a news conference scheduled for later on Wednesday, a US government source said.

The explosions at the Boston Marathon finish line killed three people and injured 176 others in the worst attack on US soil since September 11, 2001.

The bombings as well as subsequent reports that someone tried to mail the deadly poison ricin to US President Barack Obama - the second report of such a letter in two days - have created a climate of uncertainty in the country. Nerves were jolted further by an inaccurate report on cable news network CNN that a bombing suspect had been arrested.

Shortly after CNN retracted its report of an arrest in the case, security officials at Boston's federal courthouse ordered staff, media and attorneys to evacuate due to a security scare and move at least 100 yards (91.4 meters) away, according to a Reuters reporter on the scene.

Bomb-sniffing dogs, fire engines and heavily armed and helmeted police arrived at the courthouse, which was reopened to employees an hour later.

Officials take crime scene photos two days after two explosions hit the Boston Marathon in Boston, Massachusetts April 17, 2013. [Photo/Agencies]

In Washington, authorities were investigating a letter addressed to the president after the contents preliminarily tested positive for the ricin. Authorities had intercepted a letter sent to Republican Senator Roger Wicker of Mississippi on Tuesday that also preliminarily tested positive for ricin.

The FBI said there was no indication of a connection to the Boston bomb attacks, but they reminded Americans of anthrax mail attacks the country in the wake of the September 11 attacks 12 years ago.

Three fatal victims

The bombs in Boston killed an 8-year old boy, Martin Richard; a 29-year-old woman, Krystle Campbell, and a Boston University graduate student and Chinese citizen, Lu Lingzi.

The crowded scene along the race course in central Boston on Monday was recorded by surveillance cameras and media outlets, providing investigators with significant video of the area before and after the two blasts.

Investigators search for evidence on the rooftop of a building located above the site of a bomb blast on Boylston Street two-days after multiple explosions at the Boston Marathon killed three and injured 176 in Boston, Massachusetts April 17, 2013. [Photo/Agencies]

Investigators were also searching through thousands of pieces of evidence from cellphone pictures to shrapnel pulled from victims' legs.

Based on the shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have placed homemade bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race watched by thousands of spectators.

Streets around the bombing site remained closed to traffic and pedestrians on Wednesday, with police continuing their work.

No one had claimed responsibility for the attack.

"Whether it's homegrown or foreign, we just don't know yet. And so I'm not going to contribute to any speculation on that," said US Secretary of State John Kerry, who until January was Massachusetts' senior senator. "It's just hard to believe that a Patriots' Day holiday, which is normally such time of festivities, turned into bloody mayhem."

Security officials who spoke on condition of anonymity said instructions for building pressure-cooker bombs similar to the ones used in Boston can be found on the internet and are relatively primitive.

Pressure cookers had also been discovered in numerous foiled attack plots in both the United States and overseas in recent years, including the failed bombing attempt in New York's Times Square on May 1, 2010, the officials said.