Source: Boston Marathon Bomb Made Of Bearings In Pressure Cooker

Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass.

Three people were killed, including an 8-year-old boy, and more than 170 were wounded.

A law enforcement source told CBS News that one of the explosive devices appears to have been placed in a metal pressure cooker (a metal kitchen pot with a locked down top) which had been placed in a black nylon bag or backpack.

Investigators also found pieces of an electronic circuit board possibly indicating a timer was used in the detonation of the bomb.

A law enforcement official told CBS News that the two bombs that exploded were made to look like discarded property. It is still unknown if one or both bombs were in garbage cans. One may have been on the sidewalk.

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The bombs were described as "low explosive," but with "anti-personnel" packing. This is consistent with doctors reporting shrapnel pulled from victims. The official said there were apparently things like BB's, ball bearings and nails in the bombs.

While the explosions were fairly large, the bombs were made of "low explosives," the official said. Right now bomb experts do not think that plastic explosives were involved.

Federal investigators said no one had claimed responsibility for the bombings on one of the city's biggest civic holidays, Patriots Day. But the blasts raised the specter of another terrorist attack on U.S. soil.

On Capitol Hill, Defense Secretary Chuck Hagel called the bombings "a cruel act of terror" and said "a thorough investigation will have to determine whether it was planned or carried out by a terror group, foreign or domestic."

President Barack Obama called the bombings a "heinous and cowardly act" and said the incident was being investigating as an "act of terrorism."

Across the U.S., from Washington to Los Angeles, police tightened security, monitoring landmarks, government buildings, transit hubs and sporting events.

"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," said Richard DesLauriers, FBI agent in charge in Boston. He said investigators had received "voluminous tips," were interviewing witnesses and were analyzing the crime scene.

Gov. Deval Patrick said contrary to earlier reports, no unexploded bombs were found. He said the only explosives were the ones that went off.

Across the U.S., from Washington to Los Angeles, police tightened security, monitoring landmarks, government buildings, transit hubs and sporting events.

The FBI took charge of the investigation, converging on a home in the suburb of Revere on Monday night. Authorities gave no details on the search. Investigators were seen leaving a building there early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.

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CBS News senior correspondent John Miller reports the man being questioned is a Saudi national currently under guard at an area hospital. He was seen running from the explosion, and a civilian chased him down and tackled him. He was turned over to Boston police and was being interviewed by the FBI.

He was being cooperative and denying any involvement and it was too soon to call him a suspect.