BREAKING: Boston Marathon Bombs Made In in Pressure Cookers; Police Search Apartment

The bombs that exploded at the Boston Marathon on Monday -- killing three people, including an 8-year-old boy, and wounding more than 170 -- were made from pressure cookers, authorities said in a brief Tuesday. The crude cookers were packed with nails, metal shards, and shrapnel in order to injure the maximum amount of people.

The explosives were in 6-liter kitchen pressure cookers, which were then zipped into black duffel bags and left on the ground. The source, who wished to remain anonymous, said officials know the rough components of the bombs but are not yet sure how they were detonated. A doctor who treated some victims corroborated with the shrapnel theory, stating that the victim's wounds may have been made with ball bearings or BBs.

Investigators have gotten much of their information from viral videos and are asking for any video, audio, or photos from spectators, however grainy or 'insignificant' they may seem; they are also gathering surveillance tapes from buisnesses. Police and the FBI are going through images frame by frame for clues.

The bombs occurred about 100 yards and 10 seconds apart during the busiest part of the race, which may have been a calculated move. The majority of non-professional recreational runners cross the finish line around the four-hour mark, often with family members nearby to cheer them on.

Investigators have received hundreds of tips and announced an award for information leading to arrests. One tip may have lead them to search an apartment in Revere, a Boston suburb, late Monday night; neighbors saw them leaving with trash bags and a duffel bag. There's no word yet on whether the search had anything to do with the attack at the Marathon yesterday. Early reports of unexploded bombs turned out to be false, according to police.

Authorities do not yet know who caused the attack. No person or organization has claimed responsibility; the Pakistani Talban has denied any role.

Security has been tightened across the US at plane, train and bus stations; landmarks and government buildings; and public events such as concerts and games. Bomb-sniffing dogs checked passengers' luggage at Boston's South Station, where security was extra-tight.

At the White House, President Obama briefed the press, saying the bombings were an act of terrorism and that "the American people refuse to be terrorized".

Richard DesLauriers, FBI agent in charge in Boston, said "We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice."