"WASHINGTON (AP) -- The explosives used in the deadly Boston Marathon bombing were contained in 6-liter pressure cookers and hidden in black duffel bags on the ground, a person briefed on the investigation told The Associated Press on Tuesday.

One of the explosives contained shards of metal and ball bearings, and another contained nails, the person said.

A second person briefed on the investigation confirmed that at least one of the explosives was made out of a pressure cooker. Both spoke on condition of anonymity to discuss an ongoing investigation.

Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off victims' 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.

President Barack Obama said called the explosions a terrorist attack and said law enforcement and intelligence officials were trying to determine who was responsible. No one has claimed responsibility for the bombings.

These types of pressure cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 joint FBI and Homeland Security intelligence report. One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the intelligence report said.

"Placed carefully, such devices provide little or no indication of an impending attack," the report said.

The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any role in the Boston Marathon attack.

Law enforcement has not yet determined what was used to set off the explosives. Typically, these bombs have an initiator, switch and explosive charge, according to a 2004 warning from the Homeland Security Department about these types of explosives.

"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.

Investigators in Boston are combing surveillance tapes and pictures from Monday."