U.S. President Barack Obama signed an emergency declaration for Massachusetts on Wednesday and ordered federal aid to supplement the local response to the twin bombings that killed three and wounded more than 170 at the Boston Marathon two days earlier.

Also Wednesday, a U.S. law enforcement source said investigators have spotted a Boston Marathon bombing suspect from security video taken before the two blasts ripped through central Boston on Monday, 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 U.S. government officials said.

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

The explosions at the Boston Marathon finish were 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 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 Roger Wicker of Mississippi on Tuesday.

The FBI said late on Wednesday it had arrested Paul Kevin Curtis, of Corinth, 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 attacks, but they reminded Americans of anthrax mail attacks the country in the wake of the Sept. 11 attacks 12 years ago.

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 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 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."

Amid an outpouring of public support for the victims, 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 bombing.

Feinberg, a leading expert on disaster compensation, will serve as administrator of the One Fund Boston, Governor Deval Patrick and Mayor Thomas Menino said in a statement. The fund, unveiled on Tuesday, was seeded with a e1 million donation from the financial services firm John Hancock.

As of 5 P.M. EDT on Wednesday, the fund was up to $7 million, its sponsors said, including donations from more than 8,500 people.