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Investigators have spotted a Boston聽Marathon bombing suspect from security video taken before two blasts ripped through central Boston聽on Monday, a U.S. 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聽U.S. government聽officials said.

Police considered making an appeal to the public for more information at a news conference, a聽U.S. government聽source said, but the FBI canceled that news conference after delaying it several times, Boston聽police聽said.

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

The bombings, as well as subsequent reports that someone tried to mail the deadly poison ricin to U.S. 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 bomb聽scare that was later found to be a false alarm.

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

The FBI said late on Wednesday it had arrested Paul Kevin Curtis, of Corinth,聽Mississippi, in connection with the letters.

Earlier the agency had said there was no indication of a connection between the ricin letters and the Boston bomb聽attacks, but they reminded Americans of anthrax mail attacks the country in the wake of the Sept. 11 attacks 12 years ago.

Three fatal victims聽

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

The crowded scene along the race course in central Boston聽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 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.

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 U.S. Secretary of State聽John Kerry, who until January was聽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."

Amid an outpouring of public support for the victims,聽Kenneth Feinberg, the attorney who oversaw compensation funds for victims of the Sept. 11 attacks and the April 2007 Virginia Tech shootings, was named to administer a fund for those affected by the Boston聽bombing.

Feinberg, a leading expert on disaster compensation, will serve as administrator of the聽One Fund Boston, Governor Deval Patrick and Mayor聽Thomas Menino聽said in a statement. The fund, unveiled on Tuesday, was seeded with a $1 million donation from the financial services firm聽John Hancock.

As of 5 p.m. EDT (2100 GMT) on Wednesday, the fund was up to $7 million, its sponsors said, including donations from more than 8,500 people.

(Additional reporting by聽Mark Hosenball聽in Washington and Scott Malone in Boston; editing by Daniel Trotta, Gary Crosse and Jim Loney)